1)  Zenyatta:  Zenyatta and Mercy's healing capabilities together provide more than enough healing for a team.  Together they can effectively keep a team healed very consistently and since their ultimates serve different functions they tend to be a good pair together.

Zenyatta is a decent support healer and he excels at healing single targets from a distance.  Zenyatta's Harmony Orb is not quite as powerful as Mercy's healing beam but his damage output with his energy orbs and Discord Orb make up for that.

Normally, Mercy's slow HP regeneration is her main means of keeping her health up.  When Zenyatta puts his Harmony Orb on Mercy, it can consistently keep her alive even better and allow her to be more aggressive at flying around to different allies to heal the rest of the team effectively.  Zenyatta consists of mostly shield HP, so he can take cover behind a wall after taking a hit to quickly regenerate all his shield HP back to full.  Zenyatta can also use his ultimate to fill all his life up instantly.  This means that Mercy doesn't necessarily need to keep an eye on him too much, but Mercy can give a quick heal for him while in the midst of battle to keep his energy orbs going strong.

Zenyatta's Discord Orb can allow Mercy to make that same target even weaker by using her Damage Boost on the ally that is attacking that target, this is a strong synergy as it can quickly take that target down with all the extra increased damage output.

It is actually quite effective for Mercy to damage boost Zenyatta.  Zenyatta's energy orbs do a lot of damage and Zenyatta is usually already firing at the enemy that has received his Discord Orb.  Keep in mind that it takes skill to aim with Zenyatta, so if the player has bad aim, Mercy may be better off supporting someone else.

Following Zenyatta as Mercy is relatively safe since Zenyatta usually stays in the backlines where its safer and he can put the Harmony Orb on her along with activating Transcendence if things get too harsh.  Plus Zenyatta's Discord Orb can protect you offensively.

Mercy and Zenyatta's ultimates provide great variety as they each serve different functions on the battlefield.  Zenyatta's ultimate Transcendence provides the team with a powerful healing capabilities that enables strong pushes towards the enemy, or to counter a push done by the enemy.  While Mercy's Resurrect is great for potentially bringing back a whole dead team back to life.  The two can be done in succession for great success.

Zenyatta greatly benefits from getting Resurrected from Mercy because he is a slow moving hero and will take a while for him to get back to the fight faster after dying.  His Discord and Harmony Orbs are very useful and overall he is high on Mercy's Resurrect list.

2)  Ana:  Mercy works well with Ana because specific allies that Mercy is healing up is a great candidate for Ana's Biotic Grenade, which will provide an incredible amount of healing to that ally for a short while, this can make your team's tank or a defensive ally nearly unstoppable.

Mercy usually doesn't need to worry too much about healing up Ana, as Ana can use her Biotic Grenade to self-heal her up when she needs it.  Biotic Grenades can also provide some quick healing for Mercy in times when she is taking too much damage.  And Ana can shoot Mercy with her Biotic Rifle from a distance, which can help keep Mercy in the fight much longer and proves to be extremely beneficial.

There is no point for Mercy to damage boost Ana, as Ana's damage output is weak overall and Mercy will have better targets to support anyways.

Ana greatly benefits from Mercy's ultimate Resurrect since it normally takes Ana a while to run back to the fight due to her slow mobility.

A Nano Boosted ally (Ana's ultimate) can help protect Mercy offensively and the Nano Boosted ally can still be a viable target for Mercy to support as damage boosting a Nano Boosted ally greatly increases the damage output of that hero.

Keep in mind there is usually no point in Ana Nano Boosting Mercy herself as Mercy spends most of her time healing allies anyways. Although the increased speed of Nano Boost can allow Mercy to position herself better from enemies and allow her to survive a bit longer thanks to the reduced damage, but there will most likely be much better targets for Ana's Nano Boost.

3)  Symmetra:  Symmetra's Photon Shield compliments Mercy's healing beam very well.  Normally Photon Shields will regenerate after 3 seconds of not taking damage, but Mercy's healing beam (and other allied healing capabilities) can bypass that and start filling that extra shield HP up instantly regardless if the target is taking damage or not.  When Mercy obtains a Photon Shield, it will simply increase her survival by a slight margin, making it a bit safer for her to support enemies in the front lines.

Even though Symmetra usually stays out of harms way and half of her health consists of regenerating shield, she has poor mobility to seek health packs, so she will benefit from Mercy's healing beam occasionally.  Symmetra is generally safe most of the time for Mercy to follow since she typically stays in the backlines with her team lobbing photon orbs or putting up sentry turrets.

Mercy's damage boosting has no effect on Symmetra's Teleporter, but Damage Boosting Symmetra's photon orbs can be very powerful, especially when she is used to take out Reinhardts, since her orbs can penetrate through shields and targets.  Even damage boosting Symmetra when she is up close to enemies with her Projector Beam proves to be quite powerful as well.

Both Mercy and Symmetra's ultimates serve similar functions:  which is to get allies back to the fight faster after dying.  At first you might think that since both are able to achieve this, it makes the synergy between the two less important.  While that is true to some extent, Symmetra's Teleporter still works good with Mercy's Resurrect.  Not every hero will be picked up by Resurrect, including Mercy herself.  This is why Symmetra's Teleporter is still viable along side Mercy's Resurrect.  Having a Teleporter out can also make using Resurrect more responsive, allowing Mercy to Resurrect someone immediately when they have died, instead of waiting until more have died, since the Teleporter will most likely be there to provide the backup.

Mercy greatly benefits from using Symmetra's Teleporter due to her slow mobility to get back to the fight faster after dying and Symmetra can benefit from getting Resurrected, especially if her Teleporter is not out at the current time.  Also keep in mind that Resurrecting someone is still much faster than someone using a Teleporter to get back to the fight after dying.

4)  Lucio:  A Lucio and a Mercy together provide an incredible source of overall team healing capabilities, and it might be overkill on that as the two heroes combined do not output a whole lot of damage, so this combo lacks damage output.  Although I can see this combo working if there are other allies on the team that can make up for the missing damage output.

Lucio's heal aura is very beneficial for keeping Mercy's HP maxed more often, thus allowing her to be more aggressive at flying around to support different allies and keeping her in the game more often.  Mercy's healing Beam on Lucio can be effective occasionally, especially when his Amp it Up is on a cooldown and he just so happens to be near Mercy at the time.

Lucio's speed aura is not that effective with Mercy since her Guardian Angel ability is a fast movement ability that she can use quite frequently.  Speed aura does not increase the speed of Mercy's Guardian Angel, but speed aura can help her to dodge enemy ultimates and move out of the enemy's line of sight a bit easier when not using Guardian Angel.

Lucio's Soundwave can be effective to help keep deadly close range enemies away from Mercy, but she can also use her Guardian Angel ability to escape from deadly close range enemies as well.

Lucio's ultimate Sound Barrier works great for keeping Mercy alive during strong pushes or when Mercy is getting flanked.  Sound Barrier in general provides great variety with Mercy's Resurrect, as they serve different functions.  This is unlike Lucio and Zenyatta, where both of their ultimates serve very similar uses, or Symmetra and Mercy's ultimates are both used to get dead heroes back to the fight faster.

Even though Mercy's Resurrect is a very strong overall team ability to bring dead allies back to the fight almost instantly, it gets the least use out of Lucio.  This is because Lucio's speed aura combined with his Amp It Up ability enables him and other nearby allies to quickly get back to the fight faster after dying.  Lucio is the 3rd fastest hero of getting back to the fight faster after dying (after Tracer and Soldier 76).  Since both Lucio and Mercy are both healers, Lucio's healing will mean less for the match when he is dead.  This makes Lucio the least important one to be Resurrected.

Even then, if Lucio died and a few others, it can still be beneficial for Mercy to Resurrect these allies to bring them back to the fight virtually instantly.  But the synergy is not as strong, since Lucio can speed boost all these dead allies back to the fight relatively fast as well.

5)  Mercy:  Double Mercy is rarely seen in Overwatch and the main reason is because Mercy does not output much damage on her own and most of her powers consist of healing allies.  Having two Mercys lacks in damage output and is a bit overkill on the healing.  One of the main benefits of two Mercys is that the healing beam can stack from multiple Mercys.  This means you can have two Mercys effectively healing the same ally.  Although, damage boosting beam does not stack from multiple Mercys.

Two Mercys are generally picked when a team is a using a specific team strategy, usually involving the support of one high damage output hero.  Two Mercys following one hero can both heal the ally or damage boost and heal the ally.  If one Mercy is getting attacked the other Mercy can heal the other Mercy and the Mercy getting attacked can continue buffing the damage dealer.  This can prove to be a powerful synergy if executed correctly.

Remember that Mercy has a self heal of 25 HP per second after not taking damage, so you may not necessarily need to heal her if there are more important allies to heal/buff.  However if the other Mercy is getting targeted then healing her will be more essential.

Two Mercys off on their own are practically hopeless.  One can use the blaster gun while the other one heals/buffs, but they should be gravitating towards other allies to support instead.

Two Mercys means twice the Resurrect power.  This can be quite powerful if spaced out properly so allies can get constantly resurrected throughout a match.  Resurrecting another Mercy tends to be well worth it since it normally takes a Mercy quite awhile to get back to the fight faster after dying due to her slow mobility when there are no allies to fly to.

1)  Zarya:  I have Zarya rated as Mercy's best ally to support and for good reason.  Mercy's healing beam works incredibly well with Zarya as it will heal up Zarya fast and efficiently.  Mercy's healing beam is far more powerful than Zenyatta's Harmony Orb or Lucio's heal aura in this case.  Mercy's healing beam virtually eliminates the need for Zarya to worry about reaching for health packs.

Zarya typically doesn't go to far away from her team so she is generally safe for Mercy to follow around.  Zarya can also protect Mercy from Projected Barriers if she is being targeted or flanked by offensive enemies.  Even Zarya's personal Particle Barriers can act as a shield for Mercy to get behind to take cover.

Mercy's damage boosting beam has virtually no use with Zarya's Graviton Surge, but damage boosting Zarya weapons tends to be very powerful, especially when Zarya's weapon is charged up.

Zarya's poor mobility makes it a long time for her to run back to the fight faster after dying, so she dramatically benefits from Mercy's Resurrect to get back almost instantly.

When Zarya uses her ultimate Graviton Surge, Mercy can damage boost Zarya's charged particle shots or another ally's ultimates or abilities to quickly wipe out the enemies stuck in the surge.

2)  Reinhardt:  Reinhardt greatly benefits from Mercy's healing beam, and he is a prime candidate for Mercy's healing.

Mercy can safely get behind Reinhardt's shield to heal him when he needs it.  Getting behind Reinhardt's shield can also allow her to fire at enemies with her Caduceus Blaster since healing Reinhardt while his shield is taking damage does nothing if he is at full health.  Mercy's blaster is quite powerful and can be very effective against stationary targets like Bastion Sentries or Torbjorn Turrets.

Mercy's damage boosting can be effective with Reinhardt but only when he gets close to his enemies with his Rocket Hammer or immediately after he launches a Fire Strike, otherwise there is no point in damage boosting him.  Damage boosting Reinhardt before he does an Earthshatter will do 65 damage to all enemies instead of 50 to all enemies, which is not huge.  Remember when Reinhardt has his shield out there is no point in Mercy damage boosting him as he is unable to attack with his shield deployed.

Mercy should be cautious when Reinhardt charges into the enemy, as he may do it in a dangerous place where Mercy may be very vulnerable at.  In these cases Mercy may be better off just flying to a different ally to support.

Reinhardt is one of the most important heroes for Mercy to Resurrect.  Even though Reinhardt can do a Charge to get back to the fight faster by a slight margin, Mercy's team will greatly benefit from having Reinhardt and his shield back in the action ASAP.

3)  Winston:  Mercy's healing beam is incredibly powerful with Winston, but it can be difficult for Mercy to support Winston due to his wild frequent Leaps.  Winston can Leap in dangerous situations for Mercy, but it's not real bad thanks to Winston's Barrier Projectors, as Mercy can be safe for a short period of time under Winston's barrier.

Mercy's damage boosting gets the most use when Winston is attacking multiple enemies at once with the chained lighting effects of his Tesla Cannon, however everything else for Winston is not so effective for damage boosting.  When Winston uses his ultimate Primal Rage, there is no point in healing Winston since he will get fully healed anyways to 1000 HP.  Damage boosting Primal Rage is only slightly effective since Primal Rage is not really made for dealing damage but rather knocking enemies away.  It can also be difficult for Mercy to keep up with Winston during Primal Rage because his Leap goes on a 2 second cooldown during it.  Mercy is usually better off supporting a different ally during Winston's Primal Rage.

Even though Winston can Leap back to the fight faster after dying, he can still take advantage of Mercy's Resurrect to get back almost instantly.

Even though Mercy can use her Guardian Angel ability to avoid flankers, she can take advantage of Winston's Primal Rage to knock back enemies away from her to help her survive in harsh situations.

4)  Roadhog:  Mercy's healing is by far the single best allied healing capability in Overwatch.  Roadhog can be a great target for Mercy to heal, especially if Roadhog is being attacked by multiple enemies at once.  Keep in mind that Mercy gains ultimate charge while healing enemies, so Roadhog should refrain from using his Take a Breather if he knows Mercy can just heal him instead.  Roadhog should only use Take a Breather if Mercy is busy supporting another hero or if he is taking too much damage quickly.

Mercy's damage boost works great on Roadhog's Scrap Gun.  When Roadhog Chain Hooks an enemy into him, Mercy can switch to her damage boost to ensure the chained in victim dies from the Scrap Gun shot up close.

It can also be effective for Mercy to damage boost Roadhog's Whole Hog, but generally only if he is closer up to his enemies and the majority of the shrapnel rounds connect to his targets, otherwise damage boost is not so effective on Whole Hog.  Whole Hog's main use is to knock back enemies, not necessarily for damage.

Roadhog's extremely poor mobility makes it very hard for him to get back to the fight faster after dying, so he greatly benefits from Mercy's ultimate Resurrect to get back to the fight very fast.  Since Roadhog is one of the most self sufficient tanks, he can usually take care of himself after he gets resurrected, even if there aren't too many allies around him at the time.

5)  D.va:  Since D.va's boosters allow her to constantly fly to health packs to refill her HP up and her Defense Matrix ability can block incoming shots, she isn't a prime target for healing.  Not to mention D.va's ultimate will blow up her mech anyways and she can then call down a fresh one with full health, so healing her is not that important.  But if there are no other options, Mercy can heal D.va for easy ultimate charge and preventing D.va from worrying about boosting to health packs as much.

Mercy's damage boosting can be effective with D.va's Fusion Cannons, but remember Fusion Cannons are really only useful in close range which could expose Mercy to more danger.  Damage boosting D.va's Light Blaster gun in human form is actually very effective and is a great option for Mercy to damage boost.  Unfortunately, Mercy's damage boost does not work on D.va's Self-Destruct.

Following D.va as Mercy is generally safe because D.va's mech is a big target and can soak up a lot of damage with Defense Matrix to protect Mercy.  But Mercy needs to be careful where D.va uses her boosters at, as she may be going for health packs or trying to flank the enemy somewhere more dangerous.

Mercy's Resurrect is not overly powerful with D.va, since she can just use her boosters to quickly get back to the fight faster after dying, but resurrecting D.va is still very effective nonetheless.

1)  McCree:  Mercy works very well with McCree, her healing works tremendously great with McCree since he doesn't roam to far away from the team in general and his slow mobility to reach health packs only makes Mercy's healing even more important.  Mercy's healing tends to be more powerful over other support healers when it comes to healing McCree.

Mercy's damage boosting beam also works very well with McCree.  McCree is already a great DPS dealer, why not make him 30% more stronger with Mercy's damage boosting?  Damage boost will also allow McCree to rev up his Deadeye ultimate shots faster.

Mercy is also generally safe following McCree since he doesn't wonder too far from his team and McCree's Flashbangs will help protect Mercy from unexpected ambushes/flanks.

Resurrect is very beneficial for McCree since he is somewhat slow at getting back to the fight faster after dying and will greatly benefit from getting resurrected.  Not to mention that when McCree dies, he is usually within range of Mercy for Resurrect to work on him.

2)  Bastion:  Bastion is below average on Mercy' list of allies to heal, simply because of Bastion's Self-Heal.  Since Bastion cannot fire while using Self-Heal he can take advantage of Mercy's healing beam occasionally, especially when he is getting attacked continuously while in Sentry Form.

Mercy's damage boosting is actually incredibly powerful with Bastion.  Bastion's Gatling gun is one of the best things for Mercy to damage boost.  Bastion's gatling gun becomes extremely deadly once damage boosted, everything will seem to just "melt" almost instantly.  Bastion's ultimate Tank form can also be beneficial to damage boost as well, but Mercy should also make sure Bastion is healed up during his ultimate as he is a bit vulnerable while using it.

Bastion is somewhat safe for Mercy to follow since he typically doesn't go in the front lines as nearly as much as staying back out of too much danger.  Bastion's powerful DPS can also protect Mercy defensively.

Mercy's Resurrect is very beneficial for Bastion.  It normally takes Bastion a long time to get back to the fight due to his poor mobility.  Getting Bastion's high DPS back to the fight ASAP can be extremely beneficial.  Mercy should make sure that when Bastion gets Resurrected that he has at least some backup from other allies to protect him as his poor mobility can get him ambushed quite easily.

3)  Soldier 76:  Unlike other offensive attackers, Soldier 76 is usually in an accessible spot for Mercy to quickly fly to and provide healing or damage boosting support.  While soldier 76 can heal himself and others with Biotic Fields, Mercy can still provide quick and easy healing for Soldier 76 if he needs it.

Where Mercy really shines at with Soldier 76 is damage boosting.  In fact, I have Soldier 76's ultimate Tactical Visor as the most valuable thing in the game for Mercy to damage boost.  I also have Soldier 76's normal Pulse Rifle as the 2nd best thing to damage boost as far as normal weapons are concerned.

Soldier 76 is also very safe for Mercy to follow due to his Biotic Fields for healing and the fact that Soldier 76 typically doesn't wonder too far from his team unlike other offensive attackers.

Even though Soldier 76 can quickly sprint back to the fight after dying, he can still take advantage of Mercy's Resurrect.  Since Soldier 76 is not really a flanking or harassing hero, and he typically stays with his team, this gives Soldier 76 an easier target for Mercy's Resurrect.

4)  Pharah:  Mercy works very well with Pharah.  Mercy can easily follow Pharah while she jet packs around thanks to Mercy's Guardian Angel and Angelic Descent (air floating abilities).  Healing Pharah is very effective since she can take plenty of damage while flying around in the air.

Mercy's damage boosting is very effective with Pharah's rockets and Barrage as well.

The only negative aspect is that it can be a little dangerous for Mercy to follow Pharah around in the skies as she can be shot down by snipers or other long range damage dealers that have high long range accuracy.  But Mercy is very versatile and can quickly come to Pharah's aid to heal her and then move onto another hero quite easily, thus reducing the risk.

Pharah greatly benefits from Mercy's Resurrect.  Even though Pharah can use her Jump Jets to get back to the fight slightly faster, getting resurrected is still a huge time saver for Pharah, even more so than Symmetra's Teleporter.

5)  Junkrat:  Mercy works well with Junkrat since he doesn't wonder to far from his team like offensive attackers do.  Junkrat has okay mobility with his mine jumps which can potentially help him to reach health packs and avoid enemy fire, but Mercy's healing beam will keep his grenade spam going without delay.

Mercy is also generally safe following Junkrat since he doesn't wonder too far from his team and his Traps/Mines will help protect Mercy from unexpected ambushes/flanks.

Mercy's damage boosting beam is a deadly combination when combined with Junkrat's grenade spam.  A +30% more powerful grenade spam can make it incredibly hard for the enemy to advance forward.  However, Damage boost does not work on Junkrat's ultimate Rip-Tire.

Even though Junkrat can use Mine jumps to get back to the fight a tad faster (but only by a small margin) and his traps will remain out in the battlefield even after he dies too, he is middle on the list of heroes that benefit the most from Mercy's ultimate Resurrect.  When Junkrat dies, he is usually within range of Mercy for Resurrect to work on him.

Damage boost does not work on Junkrat's ultimate Rip-Tire.

6)  Hanzo:  Mercy works very well with Hanzo.  Her healing works tremendously great with Hanzo since he doesn't roam to far away from the team in general and his slow mobility to reach health packs only makes Mercy's healing even more important.  Mercy's healing tends to be more powerful over other support healers when it comes to healing Hanzo.

Mercy's damage boosting beam also works well with Hanzo.  Damage boosting Hanzo's Scatter Arrow shots and even regular bow shots is deadly, especially when the player is really good with Hanzo's aiming.

Hanzo is an average hero for Mercy to follow around as far as safety goes.  Hanzo doesn't play to offensively and stays back with his team more so than other attackers.  Hanzo's Sonic Arrow can warn Mercy of incoming flankers, allowing her to fly to another ally for safety in time.

Resurrect is very beneficial for Hanzo since he is somewhat slow at getting back to the fight faster after dying and will greatly benefit from getting resurrected.  Not to mention that when Hanzo dies, he is usually within range of Mercy for Resurrect to work on him.

7)  Genji:  It's hard for Mercy to keep up with Genji and it's generally not safe for Mercy to follow a Genji around since he is usually off on his own harassing the enemies from all angles.  A Swift Strike can leave Mercy hopeless sometimes with no one to fly to.

Unfortunately Mercy's Resurrect is usually out of Genji's range, and there are many times where Resurrect simply does not work on Genji because of his nature of going too far away from his team.

8)  Torbjorn:  Since Torbjorn spends a lot of time staying in one area building/repairing his turret, he is very vulnerable to damage.  His poor mobility makes it hard for him to dodge enemy fire and to seek health packs.  Torbjorn can give himself (and allies) Armor Packs which gives him +75 armor (that cannot be healed), which does provide a little extra protection until damage eats away at his native HP pool.  Torbjorn's ultimate Molten Core does NOT heal up his health pool, but it does increase his armor temporarily.  Because of his lack of means to gain health, he benefits from Mercy's (and other allies) healing capabilities.

Torbjorn is usually a safe hero for Mercy to support because Torbjorn generally stays in the backlines keeping a watch over his turret and doesn't wonder to far from his team.

Mercy greatly benefits from Torbjorn's Armor Packs and she is a prime candidate for it as well.  Most enemies try to go after Mercy first, so she tends to take a lot of damage.  Keeping Mercy alive with a constant flow of Armor Packs from Torbjorn will help keep your whole team alive.

Mercy's damage boosting beam can be effective with Torbjorn, but only when he is attacking something and not repairing his turret.  Damage boosting Torbjorn's ultimate Molten Core tends to be very effective.

Torbjorn's slow mobility makes it longer for him to run back to the fight after dying, so he does benefit a lot from Mercy's Resurrect, especially if he doesn't have his turret out at the time.

9)  Reaper:  Reaper is actually one of the least effective heroes for Mercy to maintain.  The problem is that if Mercy were to follow Reaper around, she will sometimes find herself helpless once Reaper decides to use Shadow Wraith or Shadow Step.  This makes following Reaper as Mercy a very unsafe endeavor.  Not to mention Reaper can heal himself with Soul Globes and reach health packs with the help of Shadow Wraith.  However if Reaper needs a quick heal he can ask Mercy to fly to him (if he is in a safe location) or he can Shadow Step to Mercy.  Please note that Mercy can still fly to Reaper even while he is in Wraith Form.

If Reaper happens to be safely close to Mercy, she can damage boost him to make his shotguns 30% more powerful, which can be devastating.  Mercy can also damage boost Reaper's ultimate Deathblossom as well, but sometimes healing him may be more appropriate to keep him alive instead.

Mercy's Resurrect only has a 15 meter radius, which isn't very big.  Since Reaper is usually away from his team flanking the enemy, Resurrect usually doesn't reach to him.  Even then, Shadow Step can dramatically help Reaper get back to the fight faster after Death, so Resurrect is usually not as effective with Reaper.

10)  Widowmaker:  Although Widowmaker can use her grappling hook to reach health packs a bit quicker, she still benefits greatly from Mercy's heal.  Thanks to Mercy's Guardian Angel ability, you can easily fly to where ever she is sniping at to support her.  The only issue with supporting Widowmaker as Mercy is they tend to be somewhat apart from each other most of the time.  Widowmaker can snipe at different perches and ledges, this can potentially expose Mercy to enemy fire more and it can take up to a few seconds to reach to her.

Mercy's damage boosting can be beneficial with Widowmaker's Sniper Rifle, especially right after healing her up since she is with her at the time.  But Mercy is better off not sticking around for too long, as she is needed to heal other allies most of the time anyways.

Widowmaker's ultimate Infra-Sight is not overly powerful with Mercy.  Infra-Sight can warn Mercy of incoming offensive flankers, enabling her ahead of time to quickly fly to another ally to support instead, but that's about it.

Mercy's Resurrect only has mediocre use with Widowmaker.  After dying, Widowmaker can use her Grappling hook to quickly get back in the action and start firing away from long distances with her Sniper Rifle.  Also Resurrect usually only works on Widowmaker about half of the time since they tend to be further than the 15 meter distance that Resurrect works at.

11)  Mei:  Mei is actually one of Mercy's least useful allies to support.  Mei's Cryo-Freeze is what makes her not the best target for allies to heal, and this holds true for Mercy as well.  Mei is also self sufficient thanks to her Ice Wall ability to block shots from the enemy from hitting her and also allowing her to escape from deadly situations.  Mei benefits the most from Mercy's healing beam when she is up close trying to freeze enemies, as it will enable her to keep the frost stream going without a delay from Cryo-Freeze.  Also keep in mind that Mercy generates ultimate charge when healing her allies, so sometimes Mei should just let Mercy heal her instead of using a Cryo-Freeze in certain situations.

Unfortunately, Mercy's damage boost does not allow Mei to freeze enemies any faster with her freeze blaster.

Mei can use an Ice Wall to protect Mercy while she focuses on freezing targets up close.  Mei can place her Ice Wall so Mercy can still heal her while she focus on freezing targets.

Mercy's damage boosting is only mediocre at best with Mei.  Damage boosting Mei's Freeze gun does increase the DPS but doesn't affect the freeze times.  Damage boosting her long range Icicle shots can be beneficial but are not as important as other weapons in the game.  Damage boost increases Mei's ultimate Blizzard's damage, but it does not freeze enemies any faster.

Mei is considered low on the list of allies that are safe to follow for Mercy.  A lot of this has to do with Cryo-Freeze.  When Mercy is following Mei around, a Cryo-Freeze can just leave Mercy hopeless without no one else to fly to sometimes.

Due to Mei's poor mobility to get back to the fight faster after dying (besides using Ice Wall boost shortcuts) she greatly benefits from Mercy's Resurrect.  Mei's defensive nature usually keeps her within Mercy's range for Resurrect to work on her.

12)  Tracer:  Tracer happens to be one of Mercy's most difficult allies to support.  Since Tracer is usually out harassing and flanking the enemy she is usually very far away from Mercy to take advantage of all her abilities.  Tracer's main way of healing is to Blink to a health pack, and even a small health pack will fill a large portion of her HP up.  However if Mercy is within a safe distance of Tracer, she can give her a quick heal.

Tracer is also low on Mercy's list for damage boosting.  A large part of the issue is following Tracer around as Mercy is very deadly to her.  Tracer's Blinks can leave Mercy stranded by herself with no one to fly to.  This makes following Tracer very unsafe and unpractical for Mercy.  Tracer's role is to harass and confuse the enemy to throw them off track.  A Mercy damage boosting Tracer is not as important as damage boosting other heroes.

Damage boosting Tracer's Pulse Bomb is usually unnecessary for most enemies since a Pulse Bomb normally does 400 damage (enough to kill most heroes), however it can be useful to damage boost Tracer's Pulse Bomb when used on high HP heroes like tanks.

Mercy's Resurrect has a relatively short range, and it's usually too far away from where Tracer tends to die at, thus Resurrect is usually not so effective with Tracer.