I was running to regents the other day through the random cache of hurlers, butchees and mutants and thought "Man, there's a collection of my most hated mobs in NC".
Wherever you are, whatever mobs are about, butchees/hurlers are the most annoying bastards. They attack from miles away, make the whole screen shake when they plant a grenade at your foot and have enough health to make taking them down a conscious effort no matter what level.
I don't like the mutants either because there's always one prick with a bloody nail gun knocking me about.
What's your most hated? Either because they look ugly, have annoying behaviors or some other reason.