If, I'm going a reasonable speed and someone starts tailgating me, I slow way the fuck down because you know what? Fuck you. There is no reason I should have to speed more than normal because your dumb ass is running late. Besides, tailgaters are stupid and fun to fuck with.

But to answer the OP's question:

Women drivers on their cell phones riding my ass because they can't be bothered to pay attention to what they're doing and women who don't look when they back up. Not to say that only women do this, but it seems to me that they do it far more often than men.