Most hated hero?

Personally, bastion.

Compared to other characters like Mei, Symmetra etc. Bastion has a lot of ammo, he can heal himself at a rate, he's also good on both attack and defence, there are disadvantages to him of course, but he's annoying like crazy. If you ever go against one of these, and they're in a call, and the other person gives shields to the bastion, and the team protect the bastion. Well damn, you're dead.

I know this as I used to be a bastion main. Also, most the time, bastion gets POTG, which is also annoying.