Aztec, hands down!
Montezuma is such a !%§$, always starting wars he can't possibly win, dragging himself and whomever he's fighting with (more often than not me) to the bottom of the score list for the rest of the game.
Japan and Tokugawa comes at a handsome second, but his mercantilist policy combined with his warmongering usually ends up with the other AIs teaming up against him.
On third Mansa "tech-whore" Musa always gives me a good run in the space race, and has snatched the "first to discover"-bonuses from under my nose many a time.
I haven't had much trouble with Isabella, though... Sure, she's always annoyed and uncooperative, but she's also always too small to be a threat.