Tracer is a hero with high mobility but low health who is effective at close range encounters. Heroes like Mccree can use his Flashbang and Peacekeeper to eliminate Tracer quickly, heroes like Sombra can sneak up behind Tracer and eliminate her from a safe distance, and Pharah can stay out of Tracers' effective range and eliminate her from the skies.
Landing McCree's Flashbang on Tracer will stun her momentarily, allowing him to line up a headshot with his Peacekeeper and instantly kill her. Even without his Flashbang, his Peacekeeper still does massive damage to her outside of her effective range, and will reduce her HP to a measly 10 if he shoots her in the head when she has full health. Also, if McCree manages to hit 4 of his Fan the Hammer bullets, Tracer will die.
Pharah will deal devastating damage to Tracer while simultaneously staying out of her effective range, one direct hit with her rocket will leave Tracer with a measly 30 HP. At this point, Tracer will either be forced to use her Recall or flee the fight entirely and search for a healthpack. Overall, Pharah's ability to deal large amounts of damage and effectively stay out of Tracer's Pulse Pistols' range make her a good counter.
Soldier: 76's Heavy Pulse Rifle and Helix Rockets will deal devastating damage to Tracer. In addition, he can stay in his Biotic Field to negate most of the damage Tracer can do to him. Landing Helix Rockets on Tracer will leave her with critical health. Meaning a few body shots (or 1 headshot) with his Heavy Pulse Rifle will kill her.
Sombra's Hack will disable Tracer's Blink and Recall, leaving her completely vulnerable depending on where she currently is. Due to her low health, Sombra herself can effectively kill Tracer without the assistance of her team. However, one should take into consideration that Tracer can still do massive damage up close; enough to potentially kill Sombra or at least force her to use her Translocator. It is advised that she stay at a distance while engaging Tracer. In most cases, a hacked Tracer is a dead Tracer.
Symmetra's Photon Projector locks onto Tracer, effectively rendering her mobility useless. In addition, her Sentry Turret will deal damage in addition to slowing her down. Symmetra can use her Shield Generator to give her and her allies 75 additional health. Making it harder for Tracer to kill them.
Torbjörn's Turret will deal continuous damage to Tracer; making her job a lot harder. A well-placed Turret will be safe from Tracer's Pulse Pistols, meaning she cannot harm it. Additionally, Torbjörn's Rivet Gun will also be viable against her. A well-placed Rivet Burst will deal exactly 150 damage which will instantly kill Tracer, and a headshot from his Rivet Shot will leave her with 10 HP, which is enough for 1 shot from the turret to kill her. Torbjörn can also supply his teammates with Armor Packs, making it harder for Tracer to kill them.
Tracer can sneak up behind Bastion while he is in Configuration: Sentry and damage his critical spot while his attention is fixated elsewhere. Bastion will have difficulty landing shots on Tracer while she moves around using her Blink. Since Bastion cannot move while in Sentry mode, she can easily stick him with her Pulse Bomb and instantly kill him.
Tracer has very high horizontal mobility and can quickly get up close to finish Mercy off. Mercy cannot fight back very well, because the Pulse Pistols will deal significantly more damage than the Caduceus Blaster at the normal engagement range, and Tracer can Recall any lost health if she takes notable damage.
Due to Orisa's low mobility; especially while firing her Fusion Driver, she will have difficulty landing shots on Tracer. She can bypass her Protective Barrier and damage her from behind. She can attempt to use Fortify to nullify the damage done by her Pulse Bomb but if she has already taken damage from Tracer prior, it might be enough to kill her despite the significantly reduced damage.
Since Reinhardt's role is to be an anchor for his team, he has very little means of taking care of Tracer while trying to protect his team. His Rocket Hammer will only deal a measly 75 damage to Tracer assuming he can even get close enough to her. And his Fire Strike can be easily avoided by her Blink. Eventually, Tracer would have dealt enough damage to him to immediately finish him off with her Pulse Bomb.
Due to Widowmaker's line of sight being limited by her scoped vision, she will most likely not see Tracer coming or be able to land any shots on her while she is moving. Tracer can potentially sneak behind enemy lines and eliminate Widowmaker while she is searching for targets. Her automatic fire will not be able to deal enough damage to stand a chance against Tracer.
Hanzo's Storm Bow deals too much damage to Tracer to be taken lightly. Landing at least 1 bodyshot with his arrow will leave Tracer with a measly 25 HP which can be followed up with either a Quick Melee or a low charged arrow. And since her health is lower than most other heroes, less Scatter Arrow fragments are required to kill her too. However, Hanzo is still a hero that requires great precision and failing to do anything against Tracer will result in his death.
Tracer's constant mobility makes freezing her difficult, and she can be frustrating to take down. However, if Mei does freeze her, then the tables will change drastically; it gives Mei an abundant amount of time to aim for the headshot that will instantly kill Tracer.
Due to Tracer's small hitbox, most often times, Reaper's overly large spread is too big to cause any serious damage. Also, Tracer's high burst damage is abundantly high for The Reaping to take much effect. However, if Reaper lands one good shot on her, Tracer will find herself in serious trouble.
Successfully landing Chain Hook on Tracer will guarantee a free kill since she has low health. However, if he misses his hook, then Tracer can use him as a damage farm or just finish him off on the spot. The burst damage from Pulse Pistols can weaken the strength of Roadhog's massive healing ability.
Although Tesla Cannon's auto-tracking reduces Tracer's mobility's effectiveness, against it, Tracer has a maximum of 297 effective health with Recall available, and her high close-range damage can put Winston in trouble if he doesn't play cautiously, not to mention his high HP greatly helps Tracer charge up her Pulse Bomb.
Zarya can protect her and her allies from Tracers' Pulse Bomb which will save them as well as provide Zarya with a free 40 energy charge. Also her large burst damage from her Pulse Pistols' is effective at feeding energy charge too. However, her barriers have very little health and don't last very long, meaning Tracer can still be a threat to Zarya while her Particle Barriers are on cooldown. However, if Zarya has accumulated enough energy, she can deal massive damage to Tracer. Tracer can easily escape Graviton Surge by using either her Blink or Recall abilities.
Although Tracer's mobility makes it hard for Zenyatta to hit her with his Orb of Destruction, he is still capable of dealing massive damage to her if he does hit her, especially if Orb of Discord is applied. It all comes down to how good the Zenyatta player is at aiming and adjusting to her Blink. Zenyatta's constant firing speed and damage output means it is simply too dangerous for Tracer to tackle heads on. She will always need to be cautious when engaging Zenyatta. Zenyatta can survive being stuck by her Pulse Bomb by using his Transcendence ability. However, depending on the scenario; this could prove to be very effective.
Thanks to Tracer's constant damage output and excellent mobility, she can be a necessity to her team despite her low health. Because of how much distance she covers in a map in such short time, she can potentially get ahead of her team and eliminate any high value targets such as Supports before her team arrives.
Zarya can provide Tracer with a Projected Barrier while she is engaged in combat which can be used to lessen the chances of her dying or needing to use her Recall, this is beneficial for both Zarya and Tracer since it provides Tracer with more survivability and it gives Zarya free energy charge. In addition, Zarya's Graviton Surge will group everyone up allowing Tracer to get multiple kills with her Pulse Bomb.
Since Zenyatta's Orb of Harmony is something that can be applied to someone, he can simply have it on Tracer while she advances onto the enemy lines and she can get constant heals while fulfilling her duty as a flanker.