1)  Lucio:  Lucio takes the number one spot for the best support hero to pair up with Ana and that is due to a few factors, but mostly:  Lucio's heal aura pairs very well with Ana's Biotic Grenades, as more allies will be able to take advantage of the double healing effects of the Biotic Grenade's splash radius and Lucio's aura, making this a very strong synergy.

Another factor that comes into play is Lucio's speed aura makes up for Ana's slow mobility.  With speed aura, Ana will be able to dodge enemy ultimate more effectively along with positioning herself quicker.  Keep in mind that Lucio's auras only work when you stay within 30 meters of each other and keep line of sight with him as well.

If Ana doesn't land her Sleep Dart on a close up enemy, Lucio can use his Soundwave ability to knock enemies away from her, or Lucio can use his ultimate Sound Barrier to provide protection as well.

Even though Ana's Nano Boost provides extra damage protection, allies are still somewhat vulnerable while Nano Boosted, but when combined with Lucio's ultimate Sound Barrier the Nano Boosted ally becomes an unstoppable force.  For example, Genji's Dragonblade or offensive flankers such as Tracer and Reaper Nano Boosted with a Sound Barrier is extremely powerful.

Nano Boosting Lucio himself is usually not a good idea though, as he lacks on DPS and already moves quickly on his own.  There will most likely be much better candidates for Nano Boost.

2)  Mercy:  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 shooting Mercy with the Biotic Rifle from a distance 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 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 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 Nano Boost.

3)  Zenyatta:  An ally who possesses Zenyatta's Harmony Orb is a great Target for Ana's Biotic Grenade, which will double the healing effect of the Harmony Orb, this can be very powerful synergy such as when used when offensive flankers are making their move or during pushes when used on tanks.

Since Zenyatta has a lot of rechargeable shield HP, Ana doesn't necessarily need to keep a constant eye out on Zenyatta to heal him.  Unfortunately, Biotic Grenades do not increase the recharge rate of shield HP.  Zenyatta can also save himself in time of need with his ultimate Transcendence as well.  Healing Zenyatta with Biotic Rifle shots or Biotic Grenades can be beneficial when he is getting flanked or being chased by enemies and needs a quick heal.

If Ana receives the Harmony Orb from Zenyatta, she can use a Biotic Grenade on herself to receive double healing which can really come in handy when being flanked.

Ana's Nano Boosted ally can benefit from Zenyatta's Harmony Orb, allowing the ally to withstand a lot of damage during Nano Boost, which will greatly increase the effectiveness of both abilities.

Nano Boosting Zenyatta is usually a poor choice, as Nano Boosting other support heroes tends to be inefficient due to the reduced damage output of support heroes in general and their main roles are to heal anyways and Nano Boost does not increase healing.  But Zenyatta has the highest DPS out of all the support heroes and can take better advantage of Nano Boost's increased damage output compared to other support heroes.  The extra speed and damage reduction only makes him more effective at throwing his orbs around and taking cover faster.  Nano Boosting Zenyatta's Transcendence is virtually pointless and will be a waste.

Zenyatta's Transcendence heals allies around him for 300HP per second, when combined with a Biotic Grenade, this increases to 600HP per second, this is a bit overkill on the healing and kind of becomes unnecessary.

4)  Symmetra:  Symmetra is another hero with a lot of rechargeable shield HP, and unfortunately Biotic Grenades do not increase the recharge rate of shield HP.  Symmetra can still benefit from Biotic Rifle shots to keep her in the action more often.  Biotic Grenades can still help her receive more healing in times when she needs it (such as when she is getting flanked).

Symmetra's Photon shield provides some value for Ana is it will allow Ana to withstand some extra damage from getting attacked by offensive flankers.

Ana will greatly benefit from taking Symmetra's Teleporter as it normally takes Ana a long while to run back to the fight due to her poor mobility.

Nano Boosting Symmetra is one of the weaker choices, but it can allow her to move into the front lines more efficiently to deal damage with her short range Projector Beam, which will allow her to rev up her ultimate Teleporter faster in return.  But overall, try to Nano Boost an ally that simply does more damage.

5)  Ana:  Double Ana is generally a poor choice since Biotic Grenades do not stack healing effects and two Anas provides very poor damage output together.

Although two allies Nano Boosted can make a devastating team as long as the opposing team doesn't counter it back with their ultimates.

There is virtually no point in Nano Boosting another Ana, as her DPS is very poor and she has no reason to move in the front lines to do any harassing anyways.  Nano Boost also does not increase the healing rate of Ana's abilities.

It is possible to put a sleeping target to sleep again with another sleep dart.  This can be useful if you are trying to disable a certain hero for a longer period of time easier.

1)  Roadhog:  Ana works great with Roadhog because of Biotic Grenade's use with Roadhog's self-heal (Take a Breather) ability.  When a Biotic Grenade is struck on Roadhog, it will make his Take a Breather ability fill 100% of his life up.

Roadhog's big hitbox makes it very easy for Ana to aim Biotic Rifle shots at him in case he needs some quick healing, even from long ranges.  Keep an extra eye on Roadhog when he uses his ultimate Whole Hog, as he is unable to use his self-heal during that time.  Biotic Rifle shots can keep his ultimate going longer and more effectively.

If Ana misses a Sleep Dart on a close up flanker, Roadhog can potentially save Ana by Chain Hooking the victim into him.

Nano Boosting Roadhog is generally a good idea.  Nano Boost allows Roadhog to quickly move around to line up Chain Hook throws more efficiently. The increased damage reduction and damage output allows him to be much more aggressive at doing basically what ever he wants to do without too much worry, even while he is using his ultimate Whole Hog.

2)  Reinhardt:  Reinhardt is another easy and important target for Ana's Biotic Rifle.  Reinhardt will move slow when his shield is out, which gives Ana an easy target for Biotic Grenades or Rifle shots.  Biotic Grenades thrown on Reinhardt also pairs well when other healers are healing him too.  When combined with Ana's Biotic Rifle shots, the multiple healing sources can heal him up incredibly fast allowing him to take a lot of continuous damage.

When Reinhardt uses his ultimate Earthshatter, enemies will drop to the ground.  This can be a good opportunity to put one of those dropped enemies to sleep with a Sleep Dart before they get back up.

Reinhardt ranks high for a great candidate for Ana's Nano Boost.  Reinhardt's Rocket Hammer is devastating in close range, Nano Boost allows Reinhardt to quickly get into close range to start hammering away at all his enemies with increased damage and protection at the same time.  Nano Boost can also help Reinhardt position himself faster for a more efficient Earthshatter.  Even when Reinhardt is Nano Boosted, Ana should definitely keep an eye out on him and give him Biotic heals as he needs it.

3)  Winston:  Winston can sometimes be unsafe for support heroes like Mercy to follow, due to his nature of leaping up close to his enemies.  However, Ana can stand a distance away and heal him up via Biotic Rifle shots.  But Winston is one of the harder tank heroes to aim Biotic Rifle shots at due to his wild leaps up close to the enemy.

Winston's Barrier Projector (shield globe) can shield attacks away from Ana, possibly even if she is not inside the barrier.  This can allow Ana to keep the Biotic Rifle shots going without delay.

Nano Boosting Winston is only a mediocre choice.  A Nano Boosted Winston will be annoying to deal with as he will be able to easily catch up to his enemies faster thanks to the increased speed of Nano Boost and will be able to soak up much more damage, providing his role of a tank much more effectively.

Winston's Primal Rage is a decent ultimate to Nano Boost.  Even though Primal Rage's main use is to knock enemies around, Nano Boost will make him much more mobile while using it to bat enemies around much more effectively along with dishing out more damage as well.

Sometimes you may not want to worry about healing Winston if his ultimate is ready or nearing ready, as Winston's ultimate Primal Rage heals his HP up completely while also giving him even more HP on top of that.

4)  D.va:  Healing D.va from a distance with Biotic Rifle shots is easy for Ana due to D.va's slow movement when firing and D.va's large hitbox.  This also makes D.va an easy target for Biotic Grenades as well.

Since D.va is mostly comprised of armor, she can soak up a lot of damage, especially if she is being healed from multiple sources with a Biotic Grenade on her.

If Ana is getting flanked and misses a Sleep Dart, D.va can quickly boost up to Ana and protect her with Defense Matrix or Fusion Cannons up close.

Nano Boost has only mediocre use with D.va.  Unfortunately, Nano Boost does not increase the speed of D.va's Boosters and she will still move somewhat slow when firing, so Nano Boost's speed does not help D.va much.  But the increased damage reduction and damage output can allow her to safely stay in the front lines to soak up damage and kill enemies up close with her Fusion Cannons more effectively.

Nano Boost has no effect on D.va's ultimate Self-Destruct and D.va will lose Nano Boost if she activates her ultimate.

5)  Zarya:  Zarya has the smallest hitbox out of all the tank heroes which makes it more difficult for Ana to support her with Biotic Rifle shots.  Zarya is also comprised of 50% shield, which she can regenerate back to life when damaged, but Zarya still greatly benefits from well aimed sniper heals as she is attacking in the front lines.  Like all tanks, you will want to pay special attention and make sure Zarya is healed up at all times.

When combined with another healer on the team, Biotic Grenades can allow Zarya to soak up a lot of continuous damage, but keep in mind that Biotic Grenades do not effect shield HP regeneration rate.  Friendly Biotic Grenades will still land on Zarya when her Particle Barrier is up, but not enemy Biotic Grenades.

When Ana is getting flanked and she misses a Sleep Dart, Zarya can provide a Projected Barrier to help keep Ana alive from the flanker.

When Zarya uses her ultimate Graviton Surge, the best thing for Ana to do with the surged up enemies is to throw a Biotic Grenade at them to deal some splash damage while preventing them from healing any damage.  Ana could also Nano Boost a specific ally while providing Biotic Rifle heals to that Nano Boosted ally to help take out the clumped up enemies in the surge.

Zarya ranks very high for a great candidate to Nano Boost, especially if her Particle Gun is already charged up.  Zarya will greatly benefit from all aspects of Nano Boost making her one tough beast to take down.

1)  Soldier 76:  Ana's Biotic Grenades pairs very well with Soldier 76's Biotic Fields, as when a Biotic Grenade hits allies being healed via Soldier 76's Biotic Fields will receive double the healing effects, giving your team a very powerful AOE healing source.  This works extremely well for healing tanks up quickly and keeping your team alive, especially when initiating pushes.

Healing Soldier 76 with Biotic Rifle shots can be a little tricky while Soldier 76 is sprinting, but generally he isn't too bad of a target and might need it when his Biotic Field is not up.

During Soldier 76's ultimate Tactical Visor, he will become more of a prime target for Ana to heal with Biotic Grenades and Rifle shots as he is still vulnerable while using it.

Nano Boost makes Soldier 76 a more overall effective damage dealer and a harder target to hit thanks to the increased speed and damage reduction., especially while he's using Tactical Visor.  Soldier 76's Tactical Visor Nano Boosted can be extremely hard for enemies to avoid as he will be able to quickly chase after his opponents much faster along with dealing more damage as well.

2)  Reaper:  Reaper ranks high on the synergy list with Ana mostly because of how effective Nano Boost is with him.  Nano Boost tends to be very valuable for allies who excel at doing extreme damage in close range, and Reaper is right up there at the top with that.

Reaper's ultimate Death Blossom is already very powerful, but when Nano Boosted he will be able to move a bit quicker to align his Deathblossom radius more effectively along with being able to survive better if he gets targeted by a lot of enemies.  You may also want to give him a few Biotic Rifle shots or a Biotic Grenade while he is using Deathblossom to ensure his survival.

Throwing a Biotic Grenade at Reaper while he is out flanking will allow him to pick up Soul Globes that will provide twice as much HP, which can keep him going stronger longer.  Keep an eye out on him and giving him Biotic Rifle heal shots when he needs it.  This is very beneficial because other support heroes will have a difficult time healing Reaper from a distance.  Reaper should also communicate with you to let you know he needs healed when out flanking.

Remember you can still give Reaper Biotic heals even when he is in Wraith Form.

3)  Genji:  Although it can be difficult to shoot Biotic Rifle heal shots at Genji from a distance due to his quick abilities like Swift Strike and wall climb, he greatly benefits from well aimed rifle heals from a distance, since other allied healers will have a difficult time healing him with their healing abilities.  Like other offensive allied flankers, it's good for Genji to communicate with Ana to let her know where he is at and if he needs healed.

If timed properly a Biotic Grenade thrown on Genji while he is out flanking can be very powerful when combined with another healing source such as Zenyatta's Harmony Orb or Lucio's heal aura.

One of the biggest synergies between Ana and Genji is how effective Ana's Nano Boost is with Genji's ultimate Dragonblade.  I have Dragonblade listed as the best synergistic ultimate for Ana to Nano Boost.  Normally Genji is vulnerable while using his ultimate but with increased speed and damage reduction he has little to worry about and will be able to attack enemies much more effectively while Nano Boosted.

It's usually a good idea to throw a Biotic Grenade on a Nano Boosted Dragonblade to help ensure his survival further while using it.

4)  McCree:  McCree has overall poor mobility to reach for health packs so healing him is essential.  McCree greatly benefits from Biotic Grenades, especially when combined with other allied healing abilities to help keep him alive more effectively.

McCree is overall a very strong candidate for an ally to Nano Boost.  Nano Boost allows McCree to position himself faster for more unpredictable Flash Bang throws. Along with the increased damage output of his revolver and damage reduction makes McCree an excellent choice for Nano Boost.

Nano Boosting McCree's ultimate Deadeye is not a bad choice.  Deadeye becomes stronger making it quicker for McCree to line up kill shots faster, which in turn makes it safer for him to use his ultimate.  Nano Boosting McCree's Deadeye can also make him withstand a bit more damage while he is revving up his shots while also allowing him to move slightly faster as well.

5)  Tracer:  Tracer can be difficult to support for any healer due to her nature of her abilities to harass enemies from behind.  Tracer's quick movement will make it difficult to land Biotic Rifle shots on her.  It's very beneficial for Tracer and Ana to communicate with each other so Tracer can receive proper healing when she is out flanking.  It only takes two Biotic Rifle shots to fully heal Tracer up.  It could also only take one Biotic Grenade then one Biotic Rifle shot to heal Tracer up quite a bit.

Tracer ranks high for a great candidate for Ana's Nano Boost.  Nano Boost makes Tracer's nature of being pesky much more peskier.  With the increased speed and damage reduction she will be incredibly hard to take down.  The increased damage output will allow her to do her flanking duties much more effectively.  Nano Boost can also make it easier for Tracer to line up Pulse Bomb throws more safely.  Under Nano Boost, Pulse Bombs will also do more damage, ensuring the death of high HP heroes like tanks.

6)  Pharah:  With good aim, it can be easy to heal Pharah when she is flying around in the air with Biotic Rifle shots.  It can be more effective to used scoped shots for instant hitscan bullets as I find it's easier to land shots on Pharah when she is flying around.

Biotic Grenades can be difficult to land on Pharah when she is flying in the air and it is usually much easier to land a Biotic Grenade on her when she falls back to the ground instead.

Pharah can use a Concussive Blast to knock enemies away from Ana if she is getting flanked, especially if Ana misses a Sleep Dart.

Pharah ranks okay at being a decent target for Ana's Nano Boost.  Increased rocket damage and reduced damage while Pharah is flying around makes Nano Boosting Pharah a decent option.

Pharah's ultimate Barrage is normally very deadly, but she is also very vulnerable while using it.  When Nano Boosted she can withstand more damage while casting Barrage while also ensuring enemies will die quicker as well, thus increasing the chance of her survival while Barraging.  Although the main drawback with Nano Boosting Pharah's Barrage is that she cannot take advantage of the speed boost since she will still remain stationary while casting it.

Nano Boost does increase the speed of Pharah's flying abilities in the air.

7)  Torbjorn:  Torbjorn usually stays in the backlines while maintaining his turret.  Even though he can provide himself with armor packs, his poor mobility makes it hard for him to reach for health packs quickly so he benefits from the occasional heal from allies.  Biotic Rifle heals are an easy way to keep Torbjorn healed up, allowing him to stay near his turret to guard it.

You may also see Torbjorn be a bit risky sometimes to try to pick up Scrap pieces in the front lines.  Pay special attention when he does that and make sure you give him Biotic Rifle heals as he needs it.  Biotic Grenades can also help increase his survival, especially if there is another healer on the team.

Ana can receive an armor pack from Torbjorn, this can allow Ana to survive flanks more effectively.  Armor obtained from armor packs cannot be healed and you will have to pick up a new armor pack to obtain more armor.  Luckily, Ana is usually not to far from Torbjorn and can get new armor packs easier than offensive heroes can.

Torbjorn is a decent candidate for Nano Boost.  Nano Boosting Torbjorn allows him to safely move into the front lines to pick up Scrap while also dishing out more damage as well.  If he decides to use his ultimate Molten Core with it he will get a lot more use out of it.  Even though his turret will be unaffected by the Nano Boost, Torbjorn himself will be able to do a lot of DPS while being practically invulnerable for Nano Boost's duration.

8)  Bastion:  When Bastion is in Sentry mode he can be very vulnerable to damage so he greatly benefits from a continuous stream of Biotic Rifle healing shots from Ana to help keep him alive in many situations, this can keep Bastion firing instead of using his Self-Repair.  When combined with Ana's Biotic Grenade he can withstand even more damage and it will also double his Self-Repair rate.

Nano Boosting a Bastion Sentry is not always the best idea because he will not be able to take full advantage of the speed from Nano Boost due to his stationary nature in Sentry Mode.  But the increased damage output and damage reduction can make him a much more powerful defense.  However, Bastion's Tank Form Nano Boosted is one scary tank, being able to move faster, deal more damage and withstand more damage makes Nano Boosting Bastion's Tank Form very effective.

9)  Junkrat:  Even though Junkrat can use Mine jumps to reach for health packs a bit quicker, he still greatly benefits from the occasional Biotic Rifle heals when he needs it.  Healing Junkrat keeps his grenade spam going without delay.

Junkrat can potentially protect Ana from flankers with his Trap/Mine combo by placing them at choke points leading up to Ana's sniping spot.

Nano Boosting Junkrat is a mediocre choice.  The extra speed from Nano Boost will help Junkrat place him traps/mines much quicker and the extra damage output and damage reduction turns Junkrat into a wild killing machine if put into the right hands.  Keep in mind there is no point in Nano Boosting Junkrat if he is going to use his ultimate Rip-Tire, as it has no effect on it.

10)  Hanzo:  Besides Hanzo's wall climb ability, he will have an overall slower time reaching for health packs so he greatly benefits from Ana's Biotic Rifle shots to keep him in the fight often.  Hanzo also benefits a lot from Biotic Grenades, especially if another healing source is on him while he is attacking.

Hanzo's Sonic Arrow allows his allies to see behind walls, this can prepare Ana to launch a Biotic Grenade or Sleep Dart at enemies coming around corners and can warn her of incoming flankers.

Giving a highly skilled Hanzo player a Nano Boost can be extremely deadly.  The extra speed allows Hanzo to move a bit quicker while revving up bow shots and will greatly increase his chance of overall survival.  Nano Boost also makes Hanzo's ultimate Dragonstrike more deadly with increased DPS output.

11)  Mei:  Mei can be very effective in the front lines to freeze enemies with her freeze blaster.  Ana can keep firing Biotic Rifle shots at Mei to keep her in the front lines freezing enemies non stop.  This can be especially potent when combined with a Biotic Grenade.  This can also delay Mei's Cryo-Freeze for a later time.

Frozen enemies are easy targets for Sleep Darts, but its usually a better idea to just kill off the frozen enemy instead with quick headshots.

Unfortunately, Mei's Cryo-Freeze is the only healing ability in the game that Biotic Grenades have no synergistic use with.

Since Ana has no vertical mobility abilities, she can take advantage of Mei's Ice Wall boost to reach to higher places she can normally not get to.  When higher up, Ana can be safer from certain enemies such as Reinhardt and flankers like Tracer.

Nano Boosting Mei is only a mediocre choice.  Mei's short range frost stream is a tad on the weaker DPS side and her purpose is to freeze enemies and then let her allies take the frozen enemies out.  Even though her frost stream will do a bit more damage with Nano Boost, it won't freeze enemies any faster, so her frost stream does not benefit a whole lot from Nano Boost.  However, the increased speed and damage reduction allows her to move up close to her enemies quicker to make her a harder target to take down, allowing her to possibly freeze more enemies.  Nano Boost also does not effect freeze times of Blizzard, but it will do more initial damage from Blizzard.

12)  Widowmaker:  Both Ana and Widowmaker are snipers, but they have completely different uses on the battlefield.  Widowmaker is pure damage output while Ana is mostly a support healer.

Giving Widowmaker Biotic Rifle heals when she needs it can keep her sniping shots going non-stop.  Widowmaker should communicate with you and let you know to heal her when she needs it, otherwise Widowmaker will have to grapple to a health pack which will disable her from combat for a short period of time.

Widowmaker's ultimate Infra-Sight allows Ana to launch a Biotic Grenade or Sleep Dart at enemies coming around corners and can warn her of incoming flankers.

Widowmaker is low on the list of the best candidates to Nano Boost, mostly because as a backline sniper she does not take much advantage of the increased speed or damage reduction, but the increased damage output of Nano Boost can make Widowmaker very deadly.