Snipers for sure. I always watch the kill cam whenever I get killed by a sniper (which makes me rage more) and I see that half the time he/she was only in the process of ADS'ing. I absolutely hate getting no scoped.
SMG's. Laser sight on them = unbalanced garbage. I don't mind hipfire from a shotgun because they're sacrificing their long range game. All the same, when some twit shoots me from 50 yards away without even lifting the damn Scorpion, it's just ludicrous.
These are like, third dimension glasses, you know? They don't really work, though because I feel like I'm still seeing stuff in whatever dimension we live in.
Seriously, how can you get upset when someone kills you with a shotgun? At least they had to aim the thing correctly to get the kill and if they missed, they would have been dead. It's FAR FAR less infuriating than dying from laser sight SMGS.