Genji:  Genji is relatively weak against Mei because she can use her freeze gun on Genji and it cannot be deflected.  Although Genji can use a Swift Strike to escape out of the way to prevent getting frozen.  Mei can use her Icewall to avoid Genji's attacks and Cryo-Freeze herself after Genji Swift Strikes her.  Even though Genji can jump/climb over her icewall, the icewall still slows a genji down quite a bit, thus giving Mei enough time to escape.  If Genji uses his ultimate Dragonblade, Mei can simply use Cryo-Freeze to protect herself from it.  1vs1, Genji is better off staying a distance from Mei and using Shrukens to deal damage.

Remember that Genji can use a Deflect on Mei's Blizzard drone to send it back towards Mei to act as a Blizzard for Genji's team.

Tracer:  Tracer is an easy fight for Mei.  Mei's Icewall can trap a Tracer close to Mei, making it harder for Tracer to use blink to escape, this allows Mei to then freeze Tracer with her freeze gun.  Tracer would need to use Recall in this situation in order to attempt to escape that.  If Tracer managed to get some damage off on Mei, that wont matter a whole lot since Mei can just freeze herself to regain her health back.

In close to mid range Mei can one shot Tracer to death with one Icicle shot to the head unless Tracer has either an armor pack on, Photon shield, or Sound Barrier.

Tracer's Pulse Bomb / Recall combo is not very effective on Mei, since she can just use Cryo-Freeze real quick to save her.  Although, if Mei uses her ultimate (Blizzard), Tracer can just blink away from it to avoid it.  Tracer can also Blink away while Mei has her freeze gun on her to quickly escape out of it.

D.va:  Mei has some strong counters against D.va.  D.va's Defense Matrix does not work on Mei's short range frost blaster, but it does work on her long range icicle shots.  From a distance D.va does very little damage with her Fusion Cannons, as they are most effective in close range, however D.va will need to keep her distance from Mei or she will most likely get frozen and ejected from her mech.  Up close and D.va can escape from the freeze effect by boosting away from it.

Mei's Ice Wall is a great counter to D.va's ultimate Self-Destruct.  When you see a Self-Destruct, simply throw an Ice Wall right in front of it between you and the exploding mech, this will not only save you, but your team also.  This also enables your team to keep fighting and not have to find a wall to hide behind.

Be very careful throwing your Blizzard drone out around a D.va, because she can use her Defense Matrix to absorb the Blizzard Drone rendering it useless, which will also put your ultimate status back at zero.  If you successfully employed your Blizzard, D.va can easily boost out of the area of effect to save herself from freezing.

Winston:  Winston's Tesla Cannon weapon only does damage in close range, so he would need to get close to Mei to do any damage, but Mei's freeze blaster is simply stronger.  Even if Winston uses his ultimate Primal Rage melee attacks on Mei, she can still freeze him or she can simply use Cryo-Freeze to escape it.  Ice Walls can further keep Primal Rage under control.  This means, that Winston should generally stay away from Mei.

Winston is better off staying away from Mei and focus on other heroes to kill.  Winston's Barrier Projectors are still effective at blocking Mei's Endothermic Blaster's frost stream and long range Icicle shots.  Winston can also escape from Mei's freeze gun by simply Leaping away from it.

Bastion:  Ice Walls are very effective on Bastion and it's the main reason why Mei counters him.  Ice Walls can be put up right in front of a Bastion Sentry to prevent him from shooting for awhile, this gives your offensive allies the time to make their move to go ambush him.

Although Mei's long range Icicle shots can do some decent damage to a Bastion Sentry, it's generally not enough to take him out though.  Mei herself can also attempt to ambush a Bastion Sentry via Ice Walls and Cryo-Freezes as she moves closer to the Sentry to eventually freeze him with her frost gun, although this is rare and you will still most likely need backup for this.  Mei is more of a support for the other offensive heroes to get to Bastion easier.

Blizzard is only somewhat effective against Bastion Sentries, since the slowing effect of Blizzard has no use against a stationary Bastion Sentry anyways.  Although, the Blizzard might be able to freeze the Bastion for a short period of time.

Mei is a good counter to Bastion's Tank ultimate, since you can just throw up an Ice Wall to block Bastion's tank shots.  Also, Blizzard is also very effective against Tank form as well.

Roadhog:  Ice Wall will be Mei's best friend against Roadhog.  If you happened to get chained in by Roadhog's Chain Hook, quickly put up an Ice Wall to block his shots so you can escape.  Ice Wall is also useful for blocking Roadhog's ultimate Whole Hog, which can protect your entire team from his scrap gun spam.  Although Whole Hog will be able to kill the entire wall before the duration is up for it, it should still give your team enough time to take some cover from it.

Roadhog's large hitbox and slow movement, makes him an easy target for Mei's long range Icicle shots, and since Roadhog has really poor mobility, Blizzard happens to work really well on him.  Blizzard can also freeze Whole Hog, which is another method of disabling Roadhog's ultimate (besides Ice Wall).

Roadhog's self healing ability (Take a Breather) can be canceled by freezing him either with a Blizzard or your freeze blaster.

Reaper:  Although Mei is great at dealing with enemies in short range with her freeze blaster, it's simply not enough to deal with Reaper's short range shotgun DPS.  Reaper can use Wraith Form to prevent himself from getting frozen, but he will most likely kill Mei before that even happens.

Being flanked from behind by a Reaper is Mei's worst nightmare.  As Mei, do your best to keep away from Reaper by using Ice Walls to block his path and be ready to use Cryo-Freeze in time to heal up.

Cryo-Freeze can save Mei from Reaper's ultimate Death Blossom, but if it's still on a cooldown, you will need to try to quickly throw up an Ice Wall to block it as fast as possible.

Junkrat:  Junkrat has ways to counter all of Mei's abilities.  Junkrat can easily fire his grenades over Mei's Ice Wall dealing lots of damage safely.  Rip-Tires can also climb over her Ice Walls and kill enemies behind it.

If Mei uses a Cryo-Freeze, Junkrat can then put a Steel Trap down next to her, and she will be trapped after she unfreezes.  Junkrat can also take this a step further by placing a mine under the Cryo-Freeze as well, dealing even more damage (the deadly Trap/Mine combo).  This is one of those rare instances where pressing "RM" while Cryo-Freeze is active to cancel it makes sense since it can prevent Junkrat from putting his Trap/Mine next to you.

If Mei gets too close to Junkrat and tries to freeze him, Junkrat can just use a Concussion Mine jump to escape it.  Same thing with Mei's ultimate Blizzard, Junkrat can simply do a Concussion Mine jump to get out of it.

Pharah:  Mei is deadly close up with her freeze gun, but since Pharah can just fly up away from her, Mei is not much of a threat to Pharah.  If Mei uses her ice wall, Pharah can just boost up in the air and shoot over the ice wall, thus making Mei's ice wall less useful against a Pharah.  Mei's ultimate Blizzard tends to also be less useful against Pharah since she is always high up in the air to avoid it.  Mei's icicle shots may be able to penetrate a Pharah from long range, but even then it's not that powerful against her.

Ice Walls can be used to block Pharah's ultimate Barrage though.

Zarya:  When dealing with Mei close up, Zarya can use her Particle Barrier to prevent getting frozen by Mei's freeze gun.  Zarya can also throw a Projected Barrier on one of her allies to prevent them from getting frozen too.  When Zarya's Particle Gun is near fully charged, it will be a close tie between Zarya killing Mei from full health and Mei freezing her.

If Mei uses a Cryo-Freeze during a fight with Zarya, it won't matter a whole lot since Zarya has 200 shields, which also regenerates fast, and it will keep Zarya in the fight.  Although, it can be effective to position a Cryo-Freeze so it will block Zarya's explosive shots from hitting your allies.

Since Mei is usually right in the back with her teammates, she is usually in a targetable spot for Zarya's ultimate Graviton Surge.  When surged, you can put up an Ice Wall to block incoming shots and projectiles from hitting your trapped allies in the surge.  You can also use a Cryo-Freeze to avoid any AOE damage thrown your way, while also potentially shielding any allies behind the Cryo-Freeze.  NOTE:  Using an Ice Wall to propel you upwards while stuck in a Surge will not get you out of it.

Zarya can counter Mei's Blizzard by using a Particle Barrier, and a Projected Barrier on one of her allies in the Blizzard as well.  The Particle Barriers will stop the freeze effect and let her walk out of it safely.  If a Particle Barrier is not available to her, then Zarya is out of luck due to her slow mobility.

Widowmaker:  Widowmaker rarely makes close contact with her enemies since she is normally in the back sniping, because of this she never needs to worry about getting frozen by Mei's freeze gun.  If Widowmaker does happen to get close to Mei, she can easily use her Grappling Hook to escape to a safer spot.

If you see a Widowmaker way in the back, simply throw up an Ice Wall right in front of her to block her shots temporarily.  This could force her to relocate temporarily disabling her from combat.

Widowmaker is less effected by Blizzard since she is usually by herself sniping, away from the heart of the battle.

Mercy:  Since Mercy rarely ever comes into close range with her enemies, she tends to give Mei a harder time since Mercy will rarely come into contact with Mei's freeze gun.  If you do manage to get close to Mercy and start freezing her, Mercy can use her Guardian Angel ability to fly to another ally to escape out of it.  Icicle shots can be effective but it is generally hard to aim at her since she is constantly flying around to support her allies while taking cover behind walls or objects.

If Mercy pops out in the open, Ice Walls can be used to try and isolate Mercy from her team to let your allies have an easier time taking her down.

Blizzard can be effective against Mercy, but she can use her Guardian Angel ability to escape out of it.

Lucio:  Lucio is another hero that Mei will rarely ever freeze up close, and that is thanks to Lucio's alternate fire (Soundwave), which knocks his enemies back away from him.  Lucio's ultimate Sound Barrier can save his teammates from getting frozen by Mei's freeze gun.  Lucio's overall fast movement makes him a hard target to hit for Icicle shots at long range.

Mei's Ice Wall doesn't effect Lucio too much since he can use his speed aura with Amp It Up to quickly navigate around it.

Lucio can save his team from Mei's Blizzard with his speed aura combined with his Amp it Up ability.  And he can further protect his team from Blizzard with Sound Barrier as well.

Ana:  Be careful freezing enemies up close to Ana's team, as Ana can land a well aimed Sleep Dart on you to disable you for a short period of time.  Keep in mind that when Mei is sleeping, her cooldowns for Cryo-Freeze and Ice Wall will continue, potentially giving you an easier opportunity to escape unharmed once you awaken.

Mei's Ice Walls can act as enormous shields to block Ana's Sleep Darts and Biotic Grenades from hitting your team.  Mei can also use an Ice Wall to block Ana from healing her teammates with her Biotic Rifle.

Luckily, Mei's self heal (Cryo-Freeze) is the only healing ability in the game that Biotic Grenades have no effect on as Mei becomes invulnerable while healing up in a Cryo-Freeze.

Due to Ana's poor mobility, it can be easy for her to get caught in a Blizzard, as Ana has no quick means to escape out of it quickly.

A Nano Boosted enemy (Ana's ultimate) could take out Mei in time before she freezes too many enemies.  Although, Mei can use a Cryo-Freeze to take cover from a Nano Boosted enemy.

Soldier 76:  Since Soldier 76 excels at keeping a distance from his enemies, he doesn't have to worry too much about Mei freezing him in close range.  Soldier 76 can always sprint away if he gets too close to Mei before she freezes him.  Soldier 76's Pulse Rifle is very powerful on Mei and it will give her a tough time.

Ice Walls work really good on Soldier 76.  Ice walls can block Pulse Rifle shots, especially while Soldier 76 is using his ultimate (Tactical Visor).  Ice Walls can also block incoming Helix Rockets.

When Mei uses her ultimate (Blizzard), Soldier 76 can simply sprint away from it to avoid it.

McCree:  McCree is very deadly in close range, but so is Mei.  McCree can usually pull off his flashbang and fan fire faster than Mei can slow/freeze McCree, however Mei usually has enough health after that to Cryo-Freeze herself to regenerate her life back.  When Mei unfreezes, McCree better either run in medium range or finish her off really fast or she will most likely freeze him (it only takes 2 seconds of targeting for freeze to happen).

Mei's Ice Wall helps her to block McCree's shots as well as help her to run away from him.  Mei's alternate icicle shots helps her to deal decent DPS safely at longer ranges.

Ice Wall is an excellent counter to McCree's ultimate Deadeye.  If you hear an enemy McCree yell "It's High Noon Time", quickly find him and throw an Ice Wall right in front of his vision to nullify his ultimate.

Mei:  When going head to head with another Mei, the most important thing is to avoid getting frozen from the freeze gun.  This means you must pay special attention and see who triggered who first with the freeze gun, and if you see you were behind at all, you must use either an Ice Wall to backup away from her or use a Cryo-Freeze to save yourself.

If you find yourself stuck in the middle of another Mei's Blizzard, you can use a Cryo-Freeze to prevent yourself from getting frozen by the Blizzard.

Hanzo:  Ice Walls work good on Hanzo to block his Storm Bow shots, effectively allowing your team to move closer to him without taking damage.

Close up and Hanzo is a relatively easy target to freeze, you just have to watch out for a Scatter Arrow shot to the ground in front of you, which can do a massive amount of damage.  You can somewhat avoid Scatter Arrow shots by continuously jumping when you get to close to Hanzo.  If you actually did take a Scatter Arrow shot, be ready to use Cryo-Freeze to heal up.

As Mei, do your best to get close to Hanzo as possible by using Ice Walls and Cryo-Freezes.  Make sure your team follows you and Hanzo will be forced to back away then.  If Hanzo climbs up to a high spot, remember you can use Ice Walls to propel yourself upwards to higher areas, giving you an opportunity to freeze Hanzo in those situations.

Keep in mind that an Ice Wall can be completely destroyed by one of Hanzo's Dragonstrikes after a few seconds.

Torbjorn:  Mei works as a really strong support hero against Torbjorn and his turrets.  Mei can put up an Ice Wall right in front of Torbjorn's turret to totally block the line of sight of his turret, enabling your team to get closer to the turret to easily take it out.  If you put the Ice Wall directly on top of his turret, it will simply disable it for the duration of the Ice Wall, but once the Ice Wall is over the turret will back back shooting again.  Mei's freeze blaster can also freeze a turret as well.

Other than helping her team to take out Torbjorn's turrets, there's not much else Mei can do against Torbjorn.  Since Torbjorn is off in the distance building/repairing his turrets, he rarely comes into close contact to Mei's Freeze gun, although if he does he will most likely die to it since Torbjorn has poor mobility and is unable to escape from getting frozen.

Mei's ultimate Blizzard is oftentimes not effective on Torbjorn since he is usually in the distance from Mei while taking cover with his turret.  Although Torbjorn has no escape mechanisms to get out of the Blizzard and will most likely get frozen if he is under it.

Reinhardt:  Both Reinhardt and Mei are both deadly in close range combat, but who wins?  Reinhardt has the upper hand here thanks to his powerful Rocket Hammer, which can knock down Mei resetting her freeze gun.  Reinhardt's ultimate Earthshatter is also a powerful counter to Mei's Freeze gun, which will knock her down and reset the freeze effect.  Mei's Freeze gun also cannot go through Reinhardt's shield.  Not to mention Reinhardt can Charge Mei unless she can respond fast enough with an Ice Wall to block it.

As Mei, it's generally better to get to a higher ground to avoid most of Reinhardt's abilities (except Fire Strikes), you can use your Ice Wall to get to places where Reinhardt cannot get up to.

Keep in mind that Reinhardt's Fire Strikes can strike through just about anything, but it cannot go through an Ice Wall or a Cryo-Freeze, as they will both block it.

Reinhardt can use his shield to block Mei's ultimate Blizzard drone (but not actual Blizzard) by simply facing his shield toward the hovering drone to prevent it from flying to far.  Reinhardt's shield will NOT block the actual Blizzard in any way, however Reinhardt can Charge out of the area of effect before he gets frozen.

Zenyatta:  Zenyatta's Discord Orb can be deadly on Mei, but she has ways to counter it effectively.  Keep in mind that a Cryo-Freeze will remove Zenyatta's Discord Orb from you.  If Cryo-Freeze is not available, then you can use Ice Walls to block incoming shots to avoid extra damage from it.  Using an Ice Wall will usually give you enough time to hide behind a wall to remove the orb if you need to.  Ice Walls themselves acts as a wall for Mei and her allies to get behind to remove the Orb of Discord from them.  Blizzard can also be used to counter the Discord Orb by preventing damage done to you or any other ally around you that has the Discord Orb on them.

Since Zenyatta is a support hero that stays back with his team he rarely needs to worry about coming in close contact with Mei's Freeze gun, although if he does his slow mobility will most likely get him frozen unless he uses his ultimate Transcendence to save him.  Transcendence also protects himself and his team from Mei's Blizzard.

Symmetra:  Symmetra and Mei rarely run into each other, so Mei rarely gets the opportunity to freeze Symmetra close up, although if it happens Symmetra has very poor mobility and she can't do anything to escape it.

Normally Symmetra's Photon Orbs will strike through everything in its path, but Mei's Ice Wall or Cryo-Freeze will block it and prevent it from striking through anything else.

Mei usually isn't the one who runs into Symmetra's turrets, but if she does, her freeze gun can take out the turrets and she can always use a Cryo-Freeze to prevent death.  Ice Walls can also prevent turret damage, especially if you can't spot where the turrets are fast enough.

Symmetra doesn't have to worry about Mei hunting down her teleporter, as Mei is typically not a teleporter hunter due to her slow mobility.

Blizzard happens to be useful against Symmetra due to her poor mobility to run away from it.