Personally, I think it's the cocky-type. I've had a free roam lobby where the players would say over their microphone: "Let's kill this fa**ot until he quits." only for them (4 of them, in this example) to be killed by me more times than they killed me (I ran a positive ratio, 12 - 7, 6-4, etc.).
What do you guys think is the worst type of player to grace your game? What's your worst/best experience?