I hate mudcrabs because I'm such a cheap bastard that I don't like to spend my weapon charges on them, and I don't use magic so I end up flame-shouting them. Thing is, it takes two flame shouts to kill a mudcrab, at least on master difficulty, and I have to sit there waiting for my shout to recharge while the little wanker scuttles around, trying to pinch me.
I used to hate skeevers because they'd always jump outta nowhere and give me a disase. Now that I'm a werewolf I'm disease-proof, so they're off my black list.
And last but not least, I hate the Thalmor with a burning passion. There's something about their arrogant demeanor that angries up my blood. Whenever I come across a band of Thalmor justiciars, I make it my business to chat them up and inform them I worship Talos (which I don't - I worship Kynareth for the stamina bonus
), just so they'd attack me and I'd have a reason to kill them in self-defence.