1)  Mercy:  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 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.

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

2)  Symmetra:  Symmetra's Photon Shields are beneficial for all her allies, and it can help Junkrat a lot as well.  Junkrat tends to take some quick burst damage as he peeks within view to lob his grenades, the Photon Shields can keep him healed up better allowing him to not worry about health packs as much.

Even though Junkrat can use Mine jumps to get back to the fight faster after dying, he still greatly benefits from using Symmetra's ultimate Teleporter to get back to the fight quickly.  Symmetra usually places her Teleporter in a position where Junkrat can immediately start spamming his grenades once he teleports in.

Junkrat can place his Steel Traps in places to guard Symmetra's Teleporter from getting flanked.  Junkrat can also use the Trap/Mine combo to make his defense even stronger.  Symmetra can also use her Sentry Turrets to further protect her teleporter from enemies ambushing it.  Sentry Turrets in general can also help protect Junkrat's position from flankers.

3)  Lucio:  Lucio's heal aura is very beneficial with Junkrat, not as potent as Mercy's healing beam, but it's enough to keep Junkrat from running for health packs.  Lucio's speed aura can give Junkrat 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.

The Trap/Mine combo can be effective at guarding Lucio's position from flankers.  Enemies caught in Junkrat's Steel Trap are easy targets for Lucio to shoot at with his sonic projectiles.

Lucio can use his Soundwave ability to knock back enemies away from Junkrat, this is especially useful against enemies like Mei and her freeze blaster, Reinhardt and his Rocket Hammer and Reaper's shotguns.

Lucio's Sound Barrier can allow Junkrat to be more aggressive towards the enemy, allowing him to mine jump around more safely while spamming grenades.  When using Rip-Tire and your stationary character is getting flanked, Lucio can save you by using his Sound Barrier.

4)  Zenyatta:  Zenyatta's Harmony Orb works great with Junkrat since the two are usually within the line of sight of each other, so the Orb should stay on Junkrat most of the time, and Junkrat is a great target for the Harmony Orb in general due to his slow mobility to reach health packs (besides his mine jumps).  The Harmony Orb can keep Junkrat's grenade spam going non-stop without delay.

Be mindful of which enemy Zenyatta puts his Discord Orb on and try to focus your grenade spam that direction.  Discord Orb does +50% more damage to the target that has the Discord Orb.

Zenyatta and Junkrat have a nice combo to deal with Reinhardt and his shield.  Zenyatta can put his Discord Orb on Reinhardt, then Junkrat can launch grenades in a way which can ricochet off of walls behind Reinhardt and then bounce to him.

Zenyatta is a squishy support hero with poor mobility and as such he is a high priority target for getting flanked.  Junkrat can protect Zenyatta with his Trap/Mine combo to prevent flankers from potentially reaching to him.

Zenyatta's ultimate Transcendence works similar to Lucio's Sound Barrier, which provides the ability to withstand a lot of damage for a short period of time.  This can enable Junkrat to be more aggressive at advancing towards the enemy and it can protect his character when using Rip-Tire as well.  Unfortunately, Zenyatta's Transcendence does not heal damage done to Rip-Tires.

5)  Ana:  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 from Ana 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 (Ana's ultimate) 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.

1)  Zarya:  Zarya's Projected Barrier can be used on Junkrat when he is either getting ambushed or when he decides to be more aggressive by mine jumping around his enemies, exposing himself more to enemy fire.  A Projected Barrier can also be used when Junkrat stays still while using Rip-Tire, potentially protecting him from ambushes while steering his tire around.

Rip-Tire pairs well with Zarya's ultimate Graviton Surge.  The clumped up enemies from the Graviton Surge is a great target for a Rip-Tire since the blast radius will most likely kill anyone stuck in the surge.  Zarya can also increase the chance of the tire exceeding by getting in front of it with her personal Particle Barrier to block shots from destroying the tire for a few seconds.

Another easy combo Junkrat can do with Zarya's Graviton Surge is to simply throw a Concussive Mine at the clumped up enemies to deal a lot of damage to them.  Even Junkrat's Grenade launcher is a good way to deal damage to the surged up enemies as well.

2)  Reinhardt:  Reinhardt's shield is very useful for Junkrat to stay behind to take cover and safely fire his grenades from.  You still need to watch your back from flankers such as Tracer, Reaper and Genji.  Keeping a Steel Trap behind you in this situation will help keep you and Reinhardt alive from flankers.

Enemies trapped in a Steel Trap is an easy target for Reinhardt to Charge at, or launch a Fire Strike at.  When you see Reinhardt charging towards the enemy, start spamming grenades at his location to help keep him alive from other enemies in the area.

Rip-Tire pairs well with Reinhardt's ultimate Earthshatter.  Earthshatter will drop enemies to the ground and stun them for a few seconds.  This gives Rip-Tire an easy chance to survive and prevents enemies from running away from Rip-Tire's explosion.  While initiating Rip-Tire, Reinhardt can guard Junkrat with his Rocket Hammer melee swings, Charge ability or shield.

3)  Winston:  Winston's Barrier Projector shield works very well with Junkrat, as it can allow him to spam his grenades while up close to the enemy.  If Junkrat is in trouble, Winston can leap to Junkrat's side and put down a barrier to potentially help keep Junkrat alive.

Rip-Tire can pair okay with Winston's ultimate Primal Rage.  Normally enemies will run away from a Rip-Tire, but Primal Rage can knock enemies back towards the Rip-Tire for the explosion to make a successful contact.

Winston's Primal Rage in general can help keep deadly close enemies away from Junkrat, like Mei, Reaper and Reinhardt.

4)  D.va:  D.va's Boosters can knock down enemies giving Junkrat easy targets to fire grenades at.  D.va's Boosters can also knock down deadly close up enemies like Mei, Reinhardt and Reaper from getting too close to Junkrat.  D.va's Boosters can also knock down enemies while a Rip-Tire is traveling to make it so a Rip-Tire will be much more successful at surviving and preventing targets from running away from the tire explosion.

D.va can protect Junkrat with her Defense Matrix ability when they are close to enemies.  D.va can also increase the likelihood of a Rip-Tire exceeding by blocking shots using Defense Matrix as the tire is traveling to its destination.

It's usually not the best idea to use Rip-Tire at the same time D.va uses her Self-Destruct.  This is because enemies would already be running and hiding from the Self-Destruct anyways, which will also make it easy for them to stay clear of the Rip-Tire as well.  It's usually better to wait right after the explosion of Self-Destruct to start Rip-Tire to keep enemies away.

5)  Roadhog:  Roadhog is an independent tank that doesn't have many synergies with Junkrat.

Roadhog benefits from Junkrat's Steel Traps while he uses his Take a Breather ability (his self heal), as he cannot do anything else while healing himself.  The Steel Traps can keep enemies away from him during these times.

Roadhog's ultimate Whole Hog has neutral use when combined with Rip-Tire.  Even though the knocked back enemies will have a harder time shooting down a Rip-Tire, it can make the tire have less of an impact when it explodes potentially killing less enemies and also can make the tire have to travel further to reach the knock backed enemies.

When using Rip-Tire, Roadhog can protect Junkrat by Chain Hooking enemies that are approaching your stationary character.  Roadhog can also get in front of the Rip-Tire to protect it from enemy fire.

Roadhog's Whole Hog can scatter enemies around giving Junkrat an easier time to pick off disoriented enemies with frag grenades.  Doing an aggressive mine jump during Roadhog's Whole Hog can be effective as well.

1)  Widowmaker:  Widowmaker's ultimate Infra-Sight is extremely beneficial for Junkrat.  Infra-Sight enables Junkrat to bounce his grenades into rooms and hallways to hit enemies that he can see behind walls.  This is a very efficient synergy between these two heroes.

Infra-Sight can also help Junkrat steer his Rip-Tire more effectively, since he will be able to see how many enemies are in which rooms, etc.

Both Widowmaker and Junkrat have traps they can put down to deal with offensive flankers and ambushes.  Widowmaker has Venom Mines that can be placed in similar fashion to Junkrat's traps to provide an even stronger defense to guard choke points or from enemies creeping up on your team.

2)  Hanzo:  Like Widowmaker's Infra-Sight, Junkrat greatly benefits from Hanzo's Sonic Arrows (but in a bit lesser degree).  This can allow Junkrat to aim his Frag Launcher more effectively to bounce grenades off of walls into rooms/hallways, potentially hitting enemies that he couldn't see otherwise.

Junkrat can place his Steel Trap and Concussive Mines in places to guard choke points leading up to Hanzo's position, further protecting him from offensive flankers.

Both Rip-Tire and Hanzo's ultimate Dragonstrike force enemies to run and hide.  The both combined can be a powerful method of clearing enemies away from an objective or Payload.  They can also be used to initiate a strong push.

3)  Torbjorn:  Torbjorn can provide armor packs for Junkrat, which will provide some extra protection.  Armor obtained from armor packs cannot be healed and you will have to pick up a new armor pack to obtain more armor.  Luckily, Junkrat is usually not to far from Torbjorn and you can get new armor packs easier than offensive heroes can.

Junkrat can place his traps and mines to protect Torbjorn's turret from getting ambushed by offensive attackers.  Trapped enemies are very easy for Torbjorn to shoot with his Rivet Gun.  Junkrat can also place his Concussive Mine in a spot that will propel enemies into a turret's line of sight, so the turret can deal a good amount of damage to the victim.

If Junkrat is in any kind of trouble he can do a mine jump towards Torbjorn's turrets to take cover and potentially lure enemies into the turret's line of sight.

Unfortunately, Torbjorn's ultimate Molten Core doesn't really synergize with Rip-Tire much, except it can be a powerful force when dealing with an enemy push.

4)  Soldier 76:  Even though Junkrat can use a Mine jump to get to health packs a tad faster, he still greatly benefits from Soldier 76's Biotic Fields, and he is usually right around Soldier 76 to take advantage of it when he needs healed.

Junkrat can place Steel Traps and Mines in places to protect choke points or health packs, which can provide a safe spot for Soldier 76 to run to.  Soldier 76 can possibly even lure enemies into Junkrat's traps as well.

Using Rip-Tire at the same time Soldier 76 uses his ultimate Tactical Visor is not a bad idea.  Both ultimates will make enemies run away, which can make Tactical Visor a bit safer to use, but overall this is not that great of a synergy.

5)  Mei:  Mei's Ice Wall provides a nice cover for Junkrat and his team, and Junkrat can still dish out damage by launching his grenades or mines over her Ice Wall.  Even Rip-Tires can climb Mei's Ice Walls and then detonate, which can be a very powerful strategy for keeping tires alive as they approach their destination.

Another synergy with Ice Wall is after Mei throws her Ice Wall up, put down a Trap/Mine combo right in front of the wall.  When the wall collapses the enemies will most likely try to run forward potentially getting trapped by the Trap/Mine combo.  The big ice chunks from the collapsed wall could camouflage the traps.

When Mei uses her Cryo-Freeze ability (she freezes solid while healing herself), it can provide a nice shield for Junkrat to stay behind to take cover from while still being able to spam grenades.  Mei's Cryo-Freeze can also provide a shield to protect Rip-Tires as they reach closer to the enemy.

Mei's ultimate Blizzard will slow and then freeze targets, which can be easy targets for Junkrat to fire grenades or Concussive Mines at.  Even Rip-Tire pairs well with the chilled/frozen targets from Blizzard, preventing it from getting shot down.

6)  Pharah:  Junkrat's grenade spam is actually great crowd control for Pharah to fly more safely in the air.  The enemies will tend to be more hesitant to push forward giving Pharah an easier time to survive in the air.

A trapped enemy from Junkrat's Steel Trap is a very easy target for Pharah to shoot at.

Sometimes it can be risky for Junkrat to do his Mine jumps around the map, but with Pharah flying in the air, she has his back since she will usually be able to fire at the same enemies that Junkrat jumps into.

When Junkrat uses Rip-Tire, Pharah can distract the enemy from shooting the tire down by using a Concussive Blast on them, or even attempt to use Barrage at the same time which could increase the success rate for at least one of them succeeding.

1)  McCree:  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.

2)  Genji:  Genji can help lure enemies into Junkrat's Steel Traps.  Junkrat's grenade spam can help pave the way for Genji to traverse certain parts of a map.

Junkrat's Rip-Tire usually makes enemies flee away from it, which can be a decent time for Genji to also activate his ultimate Dragonblade to slice away at the scattered enemies.  Genji can also get in front of the Rip-Tire and use Deflect to help keep it alive.

3)  Bastion:  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.

4)  Reaper:  Reaper can use Junkrat's traps for a safe place to retreat to if he is getting overwhelmed.  By doing this he could also lure the enemy into your traps, potentially giving you and Reaper an easy kill.

Since Junkrat can fire his grenades in hallways and rooms from a distance, Reaper can communicate with you to lob grenades at his location if Reaper needs 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.

5)  Tracer:  Tracer can help lure enemies into Junkrat's Steel Traps.  Junkrat's grenade spam can help pave the way for Tracer to traverse certain parts of a map.

Junkrat's Rip-Tire can make enemies scatter around to flee from the approaching tire, this can give Tracer a good opportunity to harass enemies more safely.

6)  Junkrat:  Two Junkrats would provide twice the grenade spam power as one.  This could be very beneficial when paired with either Hanzo's Sonic Arrow or Widowmaker's Infra-Sight for vision to let them bounce grenades to their enemies from behind walls.

Besides that, there really isn't many synergies between the two.  Two Rip-Tires at once really doesn't make sense, but multiple Traps laying around is always a positive thing.