1)  Mercy:  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 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 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 ultimate 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.

2)  Symmetra:  Symmetra's Photon Shields helps all her allies by providing more HP that also regenerates after not taking damage.  However, Bastion has a Self-Repair which provides healing for himself.  Photon Shield is still valuable for Bastion, but not as valuable like it is for other offensive attackers.

Symmetra's Sentry Turrets can protect choke points leading up to a Bastion Sentry to further protect him from offensive flankers.

Symmetra's Teleporter is very beneficial for Bastion to use since he is slow moving and normally takes a long time to get back to the fight after dying.  Getting Bastion's high DPS gatling gun back in the action ASAP is extremely beneficial.

3)  Ana:  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.

Ana's ultimate Nano Boost is not always the best idea for use on a Bastion Sentry 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.

4)  Zenyatta:  Zenyatta's Harmony Orb can be useful when put on a Bastion Sentry, especially when a Sentry is positioned on top of a moving Payload or more out in the open where Zenyatta can keep line of sight with Bastion better.  Even then, Bastion's Self-Heal makes a Harmony Orb less useful on him as opposed to other allies that would benefit from it better.

Zenyatta's Discord Orb on an enemy will literally make any target melt to Bastion's gatling gun in Sentry Mode.  This becomes especially helpful against tanks.

Zenyatta's ultimate Transcendence pairs well with Bastion's Tank Form.  This combination is great for initiating strong pushes and keeping Bastion alive during his ultimate.  Transcendence in general can help Bastion survive ambushes and can also block shots coming at his direction.

5)  Lucio:  Lucio's auras are not as useful with Bastion as opposed to other allies, and this is due to the stationary nature of Bastion's Sentry Mode and Self-Heal.  Speed aura has no use with a Bastion Sentry, although it can help him get back to the fight faster after death and help him reposition himself elsewhere a bit quicker.  Bastion's Self-Heal makes Lucio's heal aura less useful with Bastion, but heal aura can provide some healing while firing.

Lucio's Soundwave (knocks back enemies) can help Bastion deal with deadly close encounters from Mei's freeze blaster, McCree's flashbang, Genji's Deflect, Reinhardt, Tracer and Reaper.

Lucio's ultimate Sound Barrier helps Bastion survive in harsh situations such as when the enemy is initiating a push, or to protect him from many enemy ultimates.  Lucio's Sound Barrier also pairs well with Bastion's ultimate Tank Form.  Normally, Bastion is a bit vulnerable while using Tank Form, but with Sound Barrier he is nearly unstoppable.

4)  Reinhardt:  Reinhardt's shield can be extremely useful in front of a Bastion Sentry.  This can dramatically keep Bastion alive more often allowing you to dish out continuous damage without worrying about using Self-Heal as much.  This combo is also very powerful when used on top of a moving Payload.

Reinhardt's shield allows Bastion in Recon Mode to safely reposition elsewhere.  Bastion's Tank Form can also be protected very well by Reinhardt's massive shield.

Reinhardt's ultimate Earthshatter drops enemies to the ground stunning them, which allows Bastion to have easy targets to shoot at.  Bastion's Tank Form is also good when paired with Earthshatter, which can easily keep Bastion alive during his ultimate.

Up close and Reinhardt can protect you against flankers with his deadly Rocket Hammer.  Reinhardt's hammer knocks enemies down, which can give Bastion time to shoot at the flankers to finish them off.

2)  Winston:  Winston's Barrier Projector (shield globes) work very well with Bastion to block incoming fire power away from him.  Winston should take note where a Bastion Sentry is positioned, so he can line up his barrier to block shots away from him.  Winston can also simply Leap to a Bastion Sentry and then throw down a barrier for a more solid protection for him.

Barrier Projectors also provide nice protection while using Tank Form.

Winston's ultimate Primal Rage can be very helpful if you are getting ambushed.  Primal Rage can bat all the enemies away from you, allowing you to freely fire at them with your gatling gun.

1)  Zarya:  Zarya's Projected Barrier can protect Bastion from getting flanked.  Zarya can also time a Projected Barrier on a Bastion Sentry before he gets hit by Reinhardt's Fire Strike or Symmetra's Photon Orbs.  Bastion can also perform his Self-Heal while being attacked under a Projected Barrier without having it get interrupted.

Zarya can also position herself in front of a Bastion Sentry while she uses a personal Particle Barrier to protect Bastion from getting shot instead.

Zarya's ultimate Graviton Surge (clumps enemies up together) happens to be a decent target for a Bastion Sentry's gatling gun to fire at.  Bastion's Tank Form is also another method of killing enemies stuck in a Graviton Surge thanks to the massive splash damage done by Tank Form's cannons.  A Projected Barrier can also be used on Bastion's Tank Form to provide extra protection to make it more successful.

3)  D.va:  If a Bastion Sentry is getting ambushed, D.va can boost to his aid to knock them down with her Boosters and then activate Defense Matrix to protect Bastion from further damage.

D.va's ultimate Self-Destruct can be used to clear the area out around Bastion, giving him an easier time to deal with his enemies.

Bastion's Tank Form can be protected by D.va's Defense Matrix or by knocking enemies down with her Boosters.

5)  Roadhog:  Roadhog is an independent tank that doesn't have many synergies with Bastion.  Although Roadhog's ultimate Whole Hog can scatter enemies around giving Bastion an easier time to pick off disoriented enemies from the knock backs of Whole Hog and can make it safer to use Tank Form in this situation as well.

With Roadhog's large hitbox, he can get in front of a Bastion Sentry to block shots from hitting him, but this can also make it harder to aim at enemies since it will be harder to see what's in front of you.

Roadhog can Chain Hook enemies into him, which can allow Bastion to get some easy shots on the chained in victim.

1)  Mei:  Mei can provide some protection for a Bastion Sentry through multiple means.  Mei can put an Ice Wall right underneath a Bastion Sentry to lift him up, leaving offensive flankers down in the dust.  This tactic is especially deadly to enemies like Reinhardt and his deadly Rocket Hammer, an enemy Mei and her freeze blaster and other enemies like Reaper and McCree.

Mei can also freeze enemies up close with her freeze gun to keep those same enemies away from a Bastion Sentry.  Frozen enemies are also very easy to shoot at for Bastion.

Mei has a powerful strategy with Bastion.  Mei can lift Bastion up with her Ice Wall to places where Bastion normally can't get to.  Bastion can then setup a Sentry in one of these high places.  This can provide further protection from offensive flankers and also helps a lot to stay away from deadly Reinhardts.  Since Reinhardt's abilities are mostly ground based, he has trouble dealing with enemies that are elevated high up.

Mei's ultimate Blizzard pairs very well with Bastion's Tank Form.  Blizzard will slow and then freeze enemies, giving Tank Form an easy time to shoot cannons at the frozen enemies.  Mei's Blizzard also works with a basic Bastion Sentry, allowing him to shoot frozen targets easily and quickly.

2)  Widowmaker:  One of Bastion's drawbacks is he cannot shoot at very long ranges, but Widowmaker covers long distances with her sniper rifle very well.

Widowmaker's ultimate Infra-Sight doesn't help Bastion as nearly as much as other allies, but it can help you see flankers coming up on you better along with being able to get ready to shoot at enemies before they come around a corner.  Infra-Sight also pairs well with Tank Form, as it can allow you to shoot at enemies just around corners, as the splash damage can still reach to them.

Widowmaker can place her Venom Mines in a spot that protects a choke point leading up to a Bastion Sentry.  This can provide some protection from flankers.

3)  Torbjorn:  Torbjorn can provide Armor Packs for his allies, which can provide some temporary protection.  A Bastion Sentry's stationary nature makes him very vulnerable to damage.  With Armor Packs he can survive a bit longer before needing to use his Self-Heal.  Armor from Armor Packs cannot be healed and will be lost unless a new armor pack is picked up.  Since both Bastion and Torbjorn are defensive heroes, they typically stick close together, so Bastion is usually around to obtain new armor packs from Torbjorn.  Torbjorn can also easily throw an Armor Pack right on top of a Bastion Sentry, which can make getting more armor very easy.

Torbjorn's turret can provide some decent DPS to supplement a Bastion Sentry's DPS, the two combined provide a great defense against attackers trying to push forward.  Torbjorn's ultimate Molten Core can make the defense even stronger by turning the turret to level 3, which can make it incredibly difficult for attackers to break through it.  Bastion can take this strong defense up another notch by using Tank Form at the same time Torbjorn uses Molten Core.  This strategy is especially useful when being overwhelmed with a lot of enemies close up.

4)  Hanzo:  Hanzo's Sonic Arrow works like Widowmaker's Infra-Sight, but in a much smaller radius.  The benefits are similar:  It can help you see flankers coming up on you better along with allowing you to be able to get ready to shoot at enemies before they come around a corner.  Sonic Arrow also pairs well with Tank Form, as it can allow you to shoot at enemies just around corners, as the splash damage can still reach to them.

Hanzo's ultimate Dragonstrike can pair well with Tank Form.  The Dragonstrike can make enemies scatter around making it harder for them to dodge cannon shots from Bastion's Tank Form.

5)  Junkrat:  Junkrat's grenade spam can reach places that a Bastion Sentry cannot shoot at, so more areas are covered well between the two.

Junkrat can place his traps leading up to a Bastion Sentry to protect him from offensive flankers.

6)  Reaper:  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.

7)  Soldier 76:  Bastion can provide a hefty DPS, and for any enemies that survive Bastion's gatling gun, Soldier 76 can finish it up with Pulse Rifle shots.

Soldier 76's Biotic Fields is a great healing ability he can use to heal his teammates, but it's not as useful with Bastion since he has his own powerful self heal.  But it can still provide some healing if Soldier 76 is close by.

Bastion's Tank Form can be okay to use at the same time with Soldier 76's ultimate Tactical Visor.  Tank Form can make enemies scatter around giving Soldier 76 a better chance of survival while using Tactical Visor.

8)  Genji:  Bastion can provide a hefty DPS, and for any enemies that live/escape it will probably be almost dead and very susceptible to Genji's Swift Strike or Shurikens.

Genji is also a good counter to a Widowmaker, which happens to be one of Bastion's most troublesome enemies to deal with.

Tank Form combined with Genji's ultimate Dragonblade can be a very powerful force together.  For any enemies that survive a cannon ball shot, Genji can finish them off with a slice from his Dragonblade.

9)  Tracer:  Tracer's harassing can put small dents in a lot of enemies, then a Bastion Sentry can finish off the damaged victims.

Tracer is a good counter to a Widowmaker, which happens to be one of Bastion's most troublesome enemies to deal with.

10)  Pharah:  Pharah can use a Concussive Blast against offensive flankers such as Genji, Tracer, and Reaper to fend them away from a Bastion Sentry.

Other than that, there isn't many other synergies the two can do together.

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

12)  Bastion:  A dual Bastion can provide twice the DPS defense coverage as only one.  Two Bastions can work very well when combined with a Reinhardt's shield, but Symmetra's Photon Orb and a Reinhardt's Fire Strike can counter this powerful defense.

When on Defense, picking two Bastions may be a good strategy, but on Attack, I cannot see this working very well.

Two Bastions using Tank Form sounds like something I do not want to go up against.