Boorish loudmouthed unempathetic people.
If you think your right to have fun is more important than another person's right not to be miserable, you'll get on my nerves. One of the guys in my circle of friends is like this, and it grates every time. For an arts student coming up on his second year of philosophy, he doesn't have a sound grasp of logic either, so arguing with him is intensely aggravating.
Me: "I'd rather you didn't splash me with the icy cold water."
Him: "Why would you be in the water if you didn't want to be covered in it." I could understand this as a joke, it'd be hilarious, but he meant this seriously. The only way to get through to him that I seriously did not want to be splashed was to threaten to punch him in the face. That's completely ridiculous. You shouldn't have to threaten people to convince you that you're serious.
Me: (Our other friend has complained about a song choice, as he dislikes it intensely) "We should switch songs."
Him: "Well, most of us seem to like it." Really, I have to explain to someone in philosophy that might doesn't make right? It's not as if there's a shortage of songs we all like. It's just ridiculously annoying how he can constantly be inconsiderate, for no real point.
He also tried to convince me I should be bisexual, because apparently he finds it good, and weird that someone else isn't interested. Explaining that I'm not attracted to guys didn't seem to do it. He kept trying to get me to eat meat, and just generally goes about in the right way to rub me the wrong way.
His habit of constantly creeping on all women doesn't help. He ends up always on their tail, and after a while, it just gets pathetic and creepy. Actually, that makes it to the top of my list: Losers who get themselves whipped by the mere existance of girls in their social group. Guys, following them, fawning over them, and the like, is really sad. Stop that.
It's not that bad for the most part, but if I'm stuck with him for more than a day, I'll be gritting my teeth by the end of it.