1)  Ana:  Reaper ranks high on the synergy list with Ana mostly because of how effective Ana's ultimate 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.  Ana may also want to give him a few Biotic Rifle shots or a Biotic Grenade while he is using Deathblossom to ensure his survival.

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

Remember, Ana can still give Reaper Biotic heals even when he is in Wraith form.

2)  Symmetra:  Symmetra's Photon Shield is extremely useful for Reaper.  Reaper randomly appears and reappears around his enemies occasionally taking small bursts of damage.  The Photon Shield help keeps Reaper healed allowing him to be more aggressive.  Normally, using Shadow Wraith will remove all debuffs (like Zenyatta's Discord Orb), but it will not remove buffs like Photon Shield.

Symmetra can put Sentry Turrets around the map, Reaper can use these traps to retreat to if he needs to get to a safe spot to be protected. Symmetra can put these traps in a room with a health pack to keep the enemies away from it while allowing offensive allied flankers like Reaper a place to go refill up on health safely.

Even though Reaper can use Shadow Wraith and Shadow Step to get back to the fight fairly fast after dying, he can still take advantage of Symmetra's Teleporter to get back to the fight even faster.  Symmetra can also put her Teleporter behind enemy lines so that Reaper is already in a flanking position once teleported.

3)  Lucio:  Lucio's speed and heal aura can work synergistically very well with Reaper.  Lucio's heal provides Reaper with a steady healing stream and will greatly reduce his need to use Shadow Wraith and to seek health packs.  Speed aura is also very beneficial for Reaper, allowing him to quickly strafe around his enemies will make it much harder for enemies to hit him.  Speed aura also works with Shadow Wraith Form as well.

In order to make this work smoothly though, both Reaper and Lucio must be at least 30 meters from each other and keep line of sight as well or Reaper will not get the auras.  Also it tends to be very effective to communicate with the Lucio player and let him know where you are at along with what you desire more, healing or speed.

If you are up close to deadly short range damage dealers like Reinhart and his Rocket Hammer or Mei and her Frost Blaster, Lucio can use his Soundwave ability to knock them back away from you with a blast of sound.

Lucio's ultimate Sound Barrier can be used at the same time with Reaper's Deathblossom to make it more successful and to make sure you survive it properly.

4)  Zenyatta:  Zenyatta's Harmony Orb can be very beneficial to Reaper but most of the time Reaper is not in Zenyatta's line of site due to his nature of being off on his own most of the time flanking his enemies, usually through passages and hallways.  If you need healing badly, you can reach Zenyatta's line of sight and ask for a quick orb heal, other than that its usually not practical to maintain a Harmony Orb on as Reaper.  Even though using Shadow Wraith will drop enemy Discord Orbs from you, it will NOT drop Harmony Orbs from you.

Sometimes you can plan a flank with Zenyatta.  For example you can plan to flank enemies like Mercy or Reinhardt.  Ask Zenyatta to put his Discord Orb on your flank target and then proceed with the flank.  Attacking enemies with a Discord orb on them will kill them very fast.  If you keep line of sight with Zenyatta you can also obtain the Harmony Orb as well to make the flank even more successful.

Zenyatta's ultimate Transcendence pairs very well with Deathblossom.  Normally Deathblossom can be risky to use, but the rapid heal of Transcendence will ensure it will be successful.

5)  Mercy:  Mercy's healing beam is very powerful, but 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 you need a quick heal you can ask Mercy to fly to you (if you are in a safe location) or you can Shadow Step to her.  Please note that Mercy can still fly to you even in Wraith Form.

If you happen to be safely close to Mercy, she can damage boost you to make your shotguns 30% more powerful, which can be devastating.  Mercy can also damage boost your Deathblossom as well, but sometimes healing may be more appropriate to keep you 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.

1)  Zarya:  Reaper is a prime target for Zarya's Projected Barrier since he is generally in the front lines attacking close to the enemy and greatly benefits from the quick temporary shield the Projected Barrier gives, allowing Reaper to dish out easy shotgun damage while blocking shots and transferring it to Zarya's Particle Cannon to make it stronger.  The Projected Barrier especially comes in handy when dealing with deadly close up enemies such as Reinhardt, Mei, and McCree.

Zarya's ultimate Graviton Surge can be paired up with Deathblossom, but it can still be risky to use Deathblossom with the surged up enemies because they can still fight back while stuck in the surge.  However obtaining a Projected Barrier (or some other kind of protection) in this situation is ideal in order to survive a Deathblossom against a Graviton Surge.

2)  Winston:  Winston's Barrier Projectors work as great crowd control for Reaper.  Winston can Leap toward Reaper while he is in the front lines flanking and then throw down a Barrier Projector.  By isolating only the flank target close to the barrier and blocking out all other firepower from the enemy can make the flank very successful.  This same tactic can also be used to protect Reaper when he is using Deathblossom.

Winston can also provide even more crowd control with his ultimate Primal Rage by knocking enemies away from Reaper, allowing Reaper to only focus on killing one at a time instead of being bombarded by a bunch at once.  Because of this, it's generally better to use Deathblossom at a different time that Winston uses Primal Rage, since Deathblossom is great for killing a lot of bunched up enemies in a small area.

3)  Reinhardt:  Reinhardt's shield is not usually useful with Reaper due to the fact that Reaper mostly roams around on his own and is usually not near Reinhardt to take advantage of his shield.  However, in certain circumstances, such as a push, Reaper can still get behind Reinhardt's shield for protection while he dishes out strong DPS with his shotguns.

Another instance where Reinhardt's shield comes in great use with Reaper is after Reinhardt uses a Charge.  Normally it can be very risky for Reinhardt to Charge his enemy due to being in a bad situation afterwards.  With Reaper's nature of being behind enemy lines, he is usually already there to have Reinhardt's back after charging.  With this in mind, both Reaper and Reinhardt should be aware where each other is at to plan successful charges.  Reaper can also Shadow Step to Reinhardt after charging to be there to get behind his shield to dish out shotgun damage.

Reinhardt's ultimate Earthshatter can allow Reaper to safely use Deathblossom to finish off the knocked down victims.

4)  D.va:  D.va can also provide crowd control support for Reaper by Boosting through enemies and knocking them away from Reaper.  D.va's ultimate Self-Destruct also acts as great crowd control for Reaper allowing him to pick off fleeing enemies and allowing you to only worry about focusing on no more than a few enemies at a time.  Sometimes a Self-Destruct will force enemies to scatter behind walls or into hallways in which Reaper can be waiting to pick off a fleeing enemy.

Before initiating Deathblossom, D.va can boost to your side and then she can use her Defense Matrix ability at the same time with Deathblossom to ensure you won't die while using it.

5)  Roadhog:  Roadhog is an independent tank that doesn't have many synergies with Reaper, although Roadhog's ultimate Whole Hog can scatter enemies around giving Reaper an easier time to pick off disoriented enemies from the knock backs of Whole Hog.  Whole Hog can also make using Deathblossom a bit safer.

When Reaper is using Deathblossom, Roadhog should keep an eye out for any Genjis in the area as that should be a prime target for him to Chain Hook and eliminate first as Genji is a great counter to Reaper's Deathblossom with his Swift Strike and Deflect combo.  Other heroes to Chain Hook while Reaper is using Deathblossom includes:  an enemy Roadhog (a Chain Hook will stop it) and McCree (his Flashbangs will stop it).

1)  Junkrat:  Be mindful where Junkrat places his Traps and Mines.  You can use his traps for a safe place to retreat to if you are getting overwhelmed.  By doing this you could also be luring the enemy into his traps, potentially giving you and Junkrat an easy kill.

Since Junkrat can fire his grenades in hallways and rooms from a distance, you can communicate with him and tell him to lob grenades in your location if you need some extra fire power backup.

Junkrat's Rip-Tire can act as great crowd control, potentially giving Reaper an easier time picking off fleeing enemies from the tire.

2)  Widowmaker:  Widowmaker's Infra-sight helps all her allies see enemies behind walls, this can let Reaper know if using Shadow Step at a certain destination is safe or not, this can also help plan Reaper's Shadow Wraith paths as well.  And overall Infra-Sight is very useful for planning flanks and keeping out of too much danger in general.

Reaper can also harass long range damage dealers that Widowmaker can have trouble with, including:  Soldier 76, Torbjorn, McCree and Hanzo.  Keeping these enemies busy dealing with Reaper will let Widowmaker freely do her dirty work.

3)  Torbjorn:  Torbjorn's Armor Packs are useful to Reaper as it can allow him to absorb more damage before needing to use a Shadow Wraith to escape out of combat to get healed.  However, Armor Packs are not as useful as Symmetra's Photon Shield, since it does not replenish itself, as you will have to pick up a new Armor Pack to gain more armor again.

Torbjorn's turret provides a nice steady DPS from a distance, giving Reaper an easier time harassing and flanking.

4)  Mei:  Reaper and Mei can be a devastating team close up to the enemy.  Mei can freeze targets with her freeze gun and Reaper can follow through with shotgun headshots to finish off the frozen victims easily.  While this is going on Mei can put up Ice Walls so they only have to fight no more than a few enemies at once.  If Mei needs healed she can just use Cryo-Freeze and Reaper can collect Soul Globes or Shadow Wraith to a health pack.

Mei's ultimate Blizzard works well with Deathblossom.  Blizzard will slow and then freeze targets, giving Reaper a safe opportunity to execute a Deathblossom.

5)  Soldier 76:  Soldier 76's Biotic Fields are powerful healing wards that he can put down underneath of him.  Since Reaper is usually off on his own harassing the enemy from behind, he is often too far away from Soldier 76 to take advantage of his Biotic Fields.  However, you can go back to Soldier 76 to get a quick heal if you need it.  You can also use Shadow Step to get back to Soldier 76 faster as well.  This is usually impractical since you can just Shadow Step to a health pack instead.

Soldier 76's ultimate Tactical Visor doesn't really have any strategic synergies with Reaper.

6)  Pharah:  Reaper excels at taking out many of Pharah's counters, including:  Bastion, Widowmaker, Hanzo, and Torbjorn.  Removing those enemies from the battlefield can let Pharah safely fly in the air to do her dirty work.

It's usually not a good idea for Pharah to use her ultimate Barrage at the same time Reaper uses Deathblossom, but Pharah can attempt to use a Concussive Blast on an enemy to knock them into a Deathblossom.

7)  Hanzo:  Hanzo's Sonic Arrows can help Reaper much in the same way that Widowmaker's Infra-Sight does, but in a much smaller radius.  Being able to see enemies behind walls lets Reaper plan his flanks and retreat more safely from a losing fight.  It can make using Shadow Step more informative and safer overall.  Sonic Arrows does not affect Reaper's shotguns much since they are hitscan (instant bullet travel).

Hanzo's ultimate Dragonstrike can make using Deathblossom a bit safer for Reaper and the scattered enemies may end up running into the Deathblossom.

8)  McCree:  Both McCree and Reaper are close up damage dealers, but Reaper is a bit more flexible at wondering further off from his team, while McCree usually stays near his team.  If both heroes work together, McCree can Flashbang enemies while Reaper has easy shots on the stunned enemies and McCree can then finish the victims off with a Fan Fire attack.

Using Deathblossom at the same time with McCree's ultimate Deadeye is not much of a synergy, but the Deathblossom can provide some distraction while McCree revs up his Deadeye shots.  This is really only recommended if there are more enemies to be killed beyond the Deathblossom radius.

9)  Bastion:  Bastion can provide a hefty DPS, and for any enemies that live/escape it will probably be almost dead and very susceptible to Reaper's shotguns.

Reaper also counters a lot of Bastions worst enemies, such as:  Symmetra, Junkrat, Hanzo, Widowmaker, Mei, and Zenyatta.

10)  Genji:  Both Reaper and Genji have similar roles of flanking and harassing the enemy, often on their own.  The main use Reaper has with Genji is that if Genji is being attacked by enemies that can bypass his Deflect (Mei, Winston, Symmetra, and Zarya), Reaper can come by his side to help take those enemies out, and especially with a Death Blossom.

The two heroes combined will have double the flanking and ambushing power together to take out snipers or other strong defenses.

When Reaper uses Deathblossom, Genji can protect him with a Deflect, by blocking off enemies from a distance.

11)  Tracer:  Both Tracer and Reaper have similar roles of flanking and harassing the enemy.  The two together don't really have much strategic synergies, but together they can provide a very aggressive offensive force to get through strong defenses, kill snipers, flank tanks, and can plan flanks on healers as well.

12)  Reaper:  Two Reapers working together is not the worst idea.  If they both make an elimination on the same target together they will each get their own Soul Globe to collect.  If one Reaper uses Wraith Form to escape from losing too much health, the other Reaper can still be in the fight to finish off the victim.

However a good counter to multiple Reapers is Pharah.  If you see the enemy has a Pharah I strongly advice against using multiple Reapers.