1)  Mercy:  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?  Mercy's 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)  Zenyatta:  Zenyatta's Harmony Orb works great with McCree since the two are usually within the line of sight of each other, so the Orb should stay on McCree most of the time, and McCree is a great target for the Harmony Orb in general due to his slow mobility to reach health packs.

Zenyatta's Discord Orb makes shooting down enemies much faster for McCree, since Zenyatta and McCree stay somewhat close to each other, the target who has the Discord Orb is usually within McCree's sight.

Zenyatta's ultimate Transcendence pairs well with McCree's Deadeye, allowing him to withstand a lot of damage while revving up Deadeye shots.  Transcendence and Deadeye is also a good combination for a strong push towards objectives and to get through choke points.

3)  Lucio:  Lucio's heal aura is very beneficial with McCree, not as potent as Mercy's healing beam, but it's enough to keep McCree from running for health packs.  Lucio's speed aura can give McCree an occasional speed boost to help him dodge enemy ultimates, or to run him back to the fight faster after dying.  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.

Lucio's ultimate Sound Barrier pairs very well with McCree's Deadeye, allowing him to withstand a lot of damage while revving up his Deadeye shots.  Lucio's Speed aura (with Amp It Up) is also very effective combined with McCree's Deadeye.  Normally McCree moves very slow with Deadeye, but with speed aura it can make it a harder target for enemies to hit him and it makes it easier for McCree to get more enemies within view to fire at.

Lucio has trouble dealing with Pharah, but McCree is a decent counter to Pharah, thus relieving some pressure off of Lucio in this case.

4)  Ana:  McCree has overall poor mobility to reach for health packs so he greatly benefits from Ana's Biotic Rifle heal shots.  McCree also greatly benefits from Ana's 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 Ana's ultimate 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 Deadeye is not a bad choice for Ana.  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.

4)  Symmetra:  Symmetra's Photon Shield is very beneficial to McCree since it's usually a hassle for McCree to reach health packs due to his poor mobility.  Photon Shields simply help keep McCree in the fight longer.

Symmetra's Sentry Turrets can slow down targets giving McCree an easier time throwing Flashbangs or simply shooting at his enemies in those situations.

McCree greatly benefits from Symmetra's Teleporter due to his slow mobility of getting back to the fight after death.

1)  Reinhardt:  Reinhardt's Barrier Field (shield) is extremely valuable to McCree.  You can safely dish out plenty of DPS behind his shield and you easily have Reinhardt's back with your Flashbang and Fan Fire combo.  With Reinhardt's shield, McCree can take on enemies that he normally would be countered by, such as:  a Bastion Sentry, Widowmaker, Torbjorn and his turret.

If you get too bombarded with enemies up close, Reinhardt's Rocket Hammer melee swings can knock them all down, giving you an easy chance to do some Fan Fire attacks to finish them off.

Reinhardt's ultimate Earthshatter is a safe time to use a Deadeye against all the knocked down enemies.

2)  Winston:  Winston's Barrier Projector is McCree's best friend.  McCree can safely dish out damage from inside Winston's Barrier Projector.  If McCree is in trouble somewhere, Winston can Leap to your rescue and then put down a barrier to further protect you from additional damage.  Winston's Barrier is also a great combination for when McCree uses Deadeye, as he is normally very vulnerable while lining up Deadeye shots.

Winston's ultimate Primal Rage with McCree's Deadeye is a mediocre combo.  Winston's Primal Rage will knock back enemies causing a lot of confusion amongst the enemy, during which a Deadeye can be a tad safer to use, and especially if Winston can put down a Barrier before using Primal Rage.

3)  Zarya:  Zarya's Projected Barrier can help keep McCree alive, especially when dealing with long range attackers such as snipers.  Up close and McCree can protect himself and Zarya with his Flashbangs.

McCree's Deadeye is not the greatest ultimate to pair with Zarya's Graviton Surge, but it can prevent the surged up enemies from running away while McCree lines up his shots.  During this time, Zarya can help protect McCree while he is using Deadeye with a Projected Barrier.  Zarya can also simply get in front of McCree to block shots from her own personal Particle Barrier.

4)  D.va:  D.va can use her Boosters to knock down enemies making it easier for McCree to Flashbang or fire at the knocked down enemies.  D.va's Defense Matrix can be used while McCree is using Deadeye for some extra protection.  Besides that there isn't much else the two can do together.  D.va's ultimate Self-Destruct does not affect Deadeye much.

5)  Roadhog:  Roadhog is an independent tank that doesn't have many synergies with McCree, although Roadhog's ultimate Whole Hog can scatter enemies around giving McCree an easier time to pick off disoriented enemies from the knock backs of Whole Hog.  Whole Hog can also be paired decently with Deadeye as the knocked back enemies will have a harder time aiming at McCree while he is revving up his Deadeye shots.

1)  Soldier 76:  Soldier 76's Biotic Fields are a godsend for McCree.  Biotic Fields are basically healing wards that McCree can get under for some quick healing.  McCree and Soldier 76 also typically stay close together so you will have a nice constant supply of Biotic fields for healing.  Soldier 76 can also Sprint back to your location to drop down a Biotic Field if you need healing badly.

Stunned enemies from McCree's Flashbangs are easy targets for Soldier 76's Helix Rockets.

Unfortunately, Soldier 76's ultimate Tactical Visor does not have much synergistic effects with Deadeye, as both ultimates are damage outputs and they provide little protection together.

2)  Widowmaker:  Widowmaker's Infra-Sight ultimate can help McCree prepare for using his Flashbangs, as you will be able to see enemies right before they come around a corner.  However, due to the instant hitscan nature of McCree's revolver, Infra-Sight is not overly powerful with McCree.

Widowmaker can place her Venom Mines in places where it can potentially protect McCree from offensive flankers, providing a little extra DPS to finish off any wondering enemies.

There is a few enemies that McCree is weak against that Widowmaker is strong against, and that includes:  Bastion, Hanzo, and Zarya.

3)  Torbjorn:  Torbjorn can provide armor packs for McCree, which will provide some extra protection.  McCree's slow mobility to reach health packs makes Armor Packs very beneficial to him.  Armor obtained from armor packs cannot be healed and you will have to pick up a new armor pack to obtain more armor.  Luckily, McCree is usually not to far from Torbjorn and can get new armor packs easier than offensive heroes can.

You can lure enemies into Torbjorn's turrets and then Flashbang them so the turrets can have some easy hits on them, you can then finish them off with a Fan Fire attack.

4)  Genji:  Together, McCree and Genji can be a decent team. McCree's Flashbangs can enable Genji to get some easy hits in.  Genji's Deflect can protect McCree while he is using Deadeye. Genji can simply get in front of McCree while he is revving up his Deadeye shots and the enemy will be highly inclined to fire at McCree during this time, so Genji can Deflect a lot of bullets right back at the enemy making it faster for McCree to line up a shot in return.

5)  Mei:  Mei can freeze targets up close and McCree can finish off the frozen targets with revolver headshots, this can allow Mei to continue to freeze more enemies without worrying about using Icicles to finish off the frozen targets.

McCree can use Mei's Cryo-Freeze as a shield to get behind to help reduce damage.  Mei's Ice Wall can provide another shield for McCree to shield damage from.

Mei's ultimate Blizzard is not overly powerful with Deadeye, but it can be an effective synergy.  Blizzard will slow enemies, but they can still fire like normal until they get frozen.  Once the enemies do get frozen they can all be easily killed anyways with regular headshots.  Unless there are a lot of enemies getting frozen, McCree is usually better off saving Deadeye for a different time.

6)  Hanzo:  Hanzo's Sonic Arrow can help McCree prepare for using his Flashbangs, as you will be able to see enemies right before they come around a corner.  However, due to the instant hitscan nature of McCree's revolver, Sonic Arrow is not overly powerful with McCree.

A Flashbanged enemy from McCree can allow Hanzo to get an easy headshot in once the victim is stunned.

Hanzo's Dragonstrike can be paired with McCree's ultimate Deadeye.  A Dragonstrike can make McCree's ultimate Deadeye more successful by confusing enemies, making it harder for them to spot McCree while he is imitating Deadeye.

7)  Bastion:  Bastion can provide a hefty DPS, and for any enemies that survive Bastion's gatling gun, McCree can finish it up with a revolver shot.

Bastion's ultimate Tank Form can be okay to use at the same time with Deadeye.  Tank Form can make enemies scatter around giving you a better chance of survival while using Deadeye.

8)  Tracer:  If McCree Flashbangs some high hitpoint heroes such as any tank, Tracer can quickly stick a Pulse Bomb on the stunned target to take the victim out.

McCree's ultimate Deadeye can make enemies run behind walls to take cover from it, this can give Tracer a good opportunity to harass the fleeing enemies.

9)  Junkrat:  If there is a Pharah on the enemy team, Junkrat will have a hard time with her, but an allied McCree will be able to take down Pharah.  Other than this, I couldn't think of any other synergies for the two heroes.

10)  McCree:  Two McCrees on a team can provide twice the revolver DPS.  Each other's Flashbangs can compliment each other with easy Fan fire attacks for the two.  Make sure you use Deadeye at a different time the other McCree uses it, otherwise you will be wasting it.  If there are many snipers/bastions on the enemy's team, you will be better off dropping one of the McCrees for a different offensive hero.

11)  Reaper:  Reaper is weak against Pharah, but McCree is a decent counter to her.  Other than this there's not much else worth mentioning between the two.

Using Reaper's ultimate Deathblossom at the same time with McCree's 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.

12)  Pharah:  While McCree uses Deadeye, Pharah can use a Concussive Blast on the enemy to disorient them from firing at McCree while revving up his Deadeye shots.  Other than that there isn't much else the two can do together.