Overwatch's Mei is an incredibly resilient defensive hero who excels at crowd-control. Her ability to freeze multiple opponents or stall pushes makes her useful in just about any situation and she is seriously one of the heroes who, at this stage, might be considered over-powered. If you like the idea of manipulating the flow of combat, Mei is the woman for the job.
Endothermic Blaster [LMB/RMB] - Mei's Endothermic Blaster fires a steady stream of vapor that will slowly begin to freeze enemy heroes, at first slowing their movement before freezing them solid for a few seconds. During that brief pause, you can press RMB to form a sharpened icicle that will fire after a short delay, which you can use to headshot your frozen prey, killing most before they even unfreeze. The icicle projectiles also have great range if you're able to compensate for the delay between pressing RMB and when they actually fire.
Ice Wall [E] - Ice Wall triggers a targeting prompt on the ground to show where the wall will be placed. Once chosen, a large frozen wall will erupt out of the ground that can be destroyed by attacks or will eventually dissipate on its own. Great at dividing enemies from the rest of their team or shutting down pushes.
Cryo-Freeze [LSHIFT] - When Mei gets into a spot of trouble and is close to dying, Cryo-Freeze will encase her in a icicle where she will recharge health and remain invulnerable for a few seconds. This ability gives Mei a ton of resilience and lets her be exceedingly aggressive. In most cases, by the time the icicle breaks and Mei is released, your own team will have caught up to reinforce you.
Blizzard [Q] - Like Zarya's Graviton Surge, Mei's Blizzard is an ultimate that is best used as a setup for a devastating finisher by another hero. She tosses a drone a short distance in front of her that begins freezing everything in a large circular area while doing steady damage over time to any enemy hero caught in the freeze.
Mei Strategy Tips
Hands down, Mei is one of my favorite heroes to play. Her ability to constantly harass the enemy from the frontlines with her Endothermic Blaster, shut down enemy movement with her Ice Wall, and then heal up when things get too intense is just too much fun. The range of her ultimate is also long enough that it's not uncommon to catch the entire team, which can lead to multi-kills if you're quick to headshot them with the icicle projectiles.
But while Mei is all that good fun, she also comes with a lot of responsibility in order to be a true asset to your team. While you'll surely get opportunities to kill more than a few foes, you're first objective is always to be controlling the flow of battle and dictating the terms of the fight. Mei can be a lot of fun, but she can also be a huge pain in the ass to her team if she's played poorly.
The first reason for that is Ice Wall, which can often hurt your own team as much as the enemy. When choosing where to place it, you need to be careful as anything less than a steady hand could cause you to misplace the wall and potential separate your own team. The obvious use for Ice Wall is to shut down corridors and passageways, giving your team precious seconds to set up before the enemy arrives, but it can also be used to divide heroes from their team. If someone pushes ahead of Reinhardt, placing an Ice Wall between them so your team can clean them up easily is always a great play.
Likewise, Mei's ultimate, Blizzard, will also require some measure of calculation to use effectively. The range of the Blizzard is large, but enemies will also have a few seconds to escape from it before being immobilized. You can use it when they've bunched together to quickly punish them, but the Blizzard can be just as effective for creating breathing room for your team. If working to hold a contested capture zone in the final seconds of a match, dropping Blizzard as a way of deterring the enemy from putting bodies on the objective is just as useful as trapping them within the freeze.