Honestly, what annoys me the most is seeing people play something they've "mastered" round after round. (Add even more cringe if they teamswap.)
We get it. You REALLY like a certain variant. But seriously - it's time to play something else for rank, money, and a new adventure.
But as for actual characters - probably BBQ Corn or "Spin to Win" Super Brainz.
I don't hate BBQ Corn because of some notion that he's OP, but annoying in the regards of people's fanatical devotion to a class that's been nerfed so very many times. It's just plain stupid. (Which ties into my earlier pet peeve of people playing him despite "mastering" the variant.)
Super Brainz isn't OP either, but it annoys me that people still just blindly charge in head-on with Turbo twister. TT is not a free 100-0 "IWIN" button anymore. Flank, use your fists/laser, THEN pop it.