5. People who make offensive (non-discirminative) jokes. I can tolerate it if they don't know there's someone it offends lurking, but if they do (e.g. joking about male rape on the male rape stamp), then I get involved.

4. People who act like they know everything.

3. People who think one thing is another.

2. People who try to offend you with something they actually know.

My absolute number 1:
People who say things about you when they don't know a thing about you, offend you because they are just a plain dickhead, and seem to not get anything you say.