1)  Symmetra:  Tracer is one of the few heroes that dramatically benefits from having Symmetra's Photon Shield.  By default, Tracer is the squishiest hero in the game with only 150 HP and no innate shields or armor.  The fact that Tracer spontaneously pops in and out of combat, she takes a lot of small bursts of damage.  Photon Shield helps to keep Tracer in the fight longer, increases her survivability and her flanking potential

Symmetra can put Sentry Turrets around the map, Tracer can use these traps to retreat to if she 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 Tracer a place to go refill up on health safely.

Even though Tracer can just Blink back to the fight fairly fast after dying, she 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 Tracer is already in a flanking position once teleported.

2)  Lucio:  As long as Tracer is within range of Lucio's speed and heal auras she will greatly benefit from both of them.  A Tracer with speed aura can be devastating to deal with, she already moves very fast with Blink and Recall, when you put Lucio's speed aura on top of that, things get crazy fast which can make shooting her down very difficult for the enemy.

Tracer also takes great advantage of Lucio's heal aura.  Lucio's heal aura tends to be even more powerful on lower hitpoint heroes since it fills up a larger percentage of health faster.  With Tracer, this will keep her healed very quickly, allowing her to avoid health packs and using Recall as much.

In order to make these auras work smoothly though, both Tracer and Lucio must be at least 30 meters from each other and also keep line of sight together or Tracer 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.

In order to make planning an ambush with Pulse Bomb more effective, you can first receive a Sound Barrier from Lucio and then proceed with the Pulse Bomb ambush.  This will let you take your time a bit better to throw out a successful bomb thanks to the increases temporary shield of Sound Barrier.

3)  Zenyatta:  Zenyatta's Harmony Orb can be very beneficial for Tracer, but most of the time Tracer is not in Zenyatta's line of site due to her nature of being off on her own most of the time flanking her 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 Tracer.

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 heals all allies around him very rapidly, this can allow Tracer to basically do what ever she wants for a short period of time without worrying about dying, as long as she sticks around the Transcendence.

4)  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 for Ana to land Biotic Rifle heal 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 from Ana to fully heal Tracer up.  It could also only take one of Ana's Biotic Grenade then one Biotic Rifle shot to heal Tracer up quite a bit.

Tracer ranks high for a great candidate for Ana's ultimate 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.

5)  Mercy:  Tracer happens to be one of Mercy's most difficult allies to support.  Since Tracer is usually out harassing and flanking the enemy she is usually very far away from Mercy to take advantage of all her abilities.  Tracer's main way of healing is to Blink to a health pack, and even a small health pack will fill a large portion of her HP up.  However if Mercy is within a safe distance of Tracer, she can give her a quick heal.

Tracer is also low on Mercy's list for damage boosting.  A large part of the issue is following Tracer around as Mercy is very deadly to her.  Tracer's Blinks can leave Mercy stranded by herself with no one to fly to.  This makes following Tracer very unsafe and unpractical for Mercy.  Tracer's role is to harass and confuse the enemy to throw them off track.  A Mercy damage boosting her is not as important as damage boosting other heroes.

Damage boosting Tracer's Pulse Bomb is usually unnecessary for most enemies since a Pulse Bomb normally does 400 damage (enough to kill most heroes), however it can be useful to damage boost Tracer's Pulse Bomb when used on high HP heroes like tanks.

Mercy's Resurrect has a relatively short range, and it's usually too far away from where Tracer tends to die at, thus Resurrect is usually not so effective with Tracer.

1)  Zarya:  Tracer is usually in a great position to take advantage of Zarya's Projected Barrier, since she is usually in the front lines harassing and ambushing the enemy.  A Projected Barrier also works great for protecting Tracer right before throwing out a Pulse Bomb, which can allow you to safely aim a good Pulse Bomb throw out..

Tracer's Pulse Bomb works great with enemies stuck in Zarya's ultimate Graviton Surge, but tracer still needs to be careful, as getting up close to the clumped up enemies is still a risky endeavor for Tracer since the enemies can still fight back while in the surge.  Tracer can Blink and then Recall after throwing the Pulse Bomb, but she can still benefit from a Projected Barrier to ensure her survival while attempting this.

2)  Winston:  Winston has the capability to Leap towards Tracer while she is harassing or flanking and then throw down a Barrier Projector for protection.  Winston can help Tracer harass enemies with Primal Rage by knocking them around.  A Primal Rage Winston combined with a Tracer zooming around can really confuse the enemy quite a bit.

3)  D.va:  D.va also has capabilities to keep up with Tracer thanks to her Boosters.  D.va can knock down enemies with her Boosters allowing Tracer to get some easy shots in.

She can protect herself and Tracer with her Defense Matrix ability, which could allow Tracer to safely throw out a Pulse Bomb without too much harm.

D.va can use a Self-Destruct to scatter enemies around giving Tracer an easier opportunity to pick off fleeing enemies.

4)  Roadhog:  Roadhog is an independent tank that doesn't have many synergies with Tracer, although Roadhog's ultimate Whole Hog can scatter enemies around giving Tracer an easier time to harass the disoriented enemies from the knock backs of Whole Hog.

Tracer's harassing distractions can make it easier for Roadhog to Chain Hook enemies into him, which in turn will give Tracer an easier time to harass with less enemies to deal with.

5)  Reinhardt:  Reinhardt's shield is not very useful with Tracer due to the fact that Tracer mostly roams around on her own and is usually not near Reinhardt to take advantage of his shield.

Reinhardt's ultimate Earthshatter can allow Tracer to safely use a Pulse Bomb to finish off the knocked down victims.

1)  Widowmaker:  Tracer's distractions allow Widowmaker to do her sniping responsibilities from afar without being targeted as much.

Widowmaker's Infra-Sight can help Tracer make better decisions on where to Blink to, or which room or hallway to go into or avoid.  You can plan Pulse Bomb throws easier with Infra-Sight as well.

2)  Torbjorn:  Torbjorn's Armor Packs are very useful to Tracer as it can allow her to absorb more damage before needing 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, which can be a nuisance for Tracer to constantly backtrack to Torbjorn for Armor Packs.  Also, normally Recall will put Tracer in the exact state she was in a few seconds ago, the only exception to this is that it will not replenish damaged armor obtained from Torbjorn's Armor Packs.

Torbjorn's turret can allow Tracer to retreat from a losing battle quicker by getting in the line of a turret's sight, thus keeping enemies away from you while you seek healing.  You can also possibly lure enemies into Torbjorn's turrets as well.

3)  Mei:  Mei can use her Ice Wall to provide crowd control for Tracer by blocking part of the enemy off from you, which can make harassing easier and more effective.

Up close and Mei can freeze targets with her freeze blaster, which can allow you to have easy headshots to finish them off.  Mei's ultimate Blizzard can chill and freeze targets as well, which can allow Tracer to get some easy Pulse Pistol shots in.

Remember if Mei uses her Cryo-Freeze ability, you can use it as a wall to take cover from.

4)  Pharah:  Even though Tracer and Pharah have very little synergies together, they are both good at countering each other's counters.

Tracer is great against these heroes that Pharah has trouble with:  Bastion, Widowmaker, Genji, and to a lesser degree, Zenyatta and Torbjorn.

While Pharah is great against these heroes that Tracer has trouble with:  Mei and Junkrat.

5)  Hanzo:  Hanzo's Sonic Arrow can help Tracer make better decisions on where to Blink to, or which room or hallway to go into or avoid.

Hanzo's ultimate Dragonstrike can make enemies flee from it, which can give Tracer an easier time to harass the enemies running away from it.

If Tracer is firing at enemies in a small room or hallway, that can signal Hanzo to fire a Scatter Arrow shot in the room with the hopes of taking someone out with it.

6)  Soldier 76:  Soldier 76's Biotic Fields can be a powerful way to heal his teammates, but tracer is an independent offensive flanker/harasser and is usually far away from her allies and Soldier 76 to take advantage of his Biotic Fields for healing.  Tracer can simply blink to health packs to provide the healing she needs.  Tracer can also use her Recall ability to regain her health back as well.

Soldier 76 and Tracer really don't have any synergies they can do together.

7)  Junkrat:  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.

8)  McCree:  If McCree Flashbangs some high hitpoint heroe such as any tank, Tracer can quickly and easily 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)  Tracer:  A double Tracer can actually be very devastating, especially when combined with Symmetra shields.  Early on in the beta Symmetra's shields provided +50 shield HP (instead of +25 like it does now).  Back then 2 Tracers and a Symmetra for providing +50 HP shields for both heroes end up being overpowered in competitive play, so they nerfed Symmetra's shields to only provide +25 HP instead.  But still even with +25 HP shields, two Tracers can be a very powerful force to harass and take out strong defenses.

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

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

11)  Genji:  Like Tracer, Genji is mostly an independent flanker and harasser.  Together they can spell disaster to strong defenses like Widowmakers and Bastion Sentries.  Besides providing double the confusion and harassing power between the two heroes, there's not much else worth mentioning.

12)  Reaper:  Like with Genji, 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.