Definitely the generic Kat combos. Any mine spamming (especially Isaac's). Raiden's aerial side circle (in FFA). Sackboy corner camping. The generic G. Cole combos. FP's minions. Everything Ratchet does.
#1: Rage quitters.
Drake's AK. The move is just annoying in all fields and even in the air.
Sweet Tooth's shotgun, both versions. I understand he is barely mid-tier but the way that shotgun is abused is downright sad & pathetic. ST players LITERALLY use this like their is no other moves existing in ST's moveset and like this is his oly vviable move.
I got into a 3 Spike match today. The other guy was a Big Daddy on my team. The other two Spikes kept using the radar...and then there's the fact I used it a few times to confirm and stuff...