YouTube pontificators. Actually, pontificators in general. I'm not talking about people who simply argue with you (because who knows, they might be right), but people who insist they're right despite mountains of evidence to the contrary, and who just cannot let it go.
I normally don't participate in these debates, but I take exception to being aggressively told that my opinion is wrong. I pointed out that they were right about A, B, and C, but that most of what they presented as fact was really a matter of opinion. I finally just threw up my hands and backed out of the argument, which was pretty much at a stalemate. You can't get anywhere with someone whose responses pretty much boil down to, "No, I'm right! I'm right, I'm right, I'm right!"