Pharah, Junkrat, and Torbjorn (especially Torb) are all good picks versus Mei.
But yeah, I can definitely see how Sombra might do against Mei. Pretty much just as well as any genji or reaper could manage.
Considering that the last time I played Overwatch I was almost exclusively playing Genji counters every round (this was before this nerf, which, thank god), so Winston & Mei and then Zarya if I actually wanted to have fun and there wasn't a mlg-pro Genji on the other team.
... We'll see how Sombra does on PTR... but I'm curious if she'll counter my waifu Zarya. Guess it depends on what the range of that hack ability is, I'm assuming it goes through shields.
... it just occurred to me, is this the Reinhardt shield counter we need? Will it still be 1 Reinhardt every round meta if you wanna win? Or does Sombra finally break that up?
... if you EMP bastion does he get taken out of turret mode?
EDIT: I'm just filled with questions today. Will Symmetra turrets target a cloaked Sombra? That would be a great stealth Symmetra buff, finally actually being able to use her turrets as "flank wards" (which I think was the original intention?), instead of just stacking 6 of them on one door for lulz. Here's hoping Symmetra just hard counters Sombra, the Symmetra buff we need.
EDIT2: I'm going to have a hilarious time typing into match chat "guys I think this Sombra is hacking!".
EDIT3: maybe I should read the thread before I post. Most of the stuff I asked seemed to be covered by @reudh in better format.