Decius. He ALWAYS shows up because of his stupid three different civs, doesn't ever seem to even have the right alignment, and doesn't follow what those civs should do anyway. It's not even "love-to-hate" with this guy, he just breaks things and annoys me too much. Raiders of course does not help...Though maybe those who know players like me get frustrated, and put him in the game anyway, are the ones who can laugh.
Though Cyrus is what I would say for vanilla since the original civ IV. In fact there've been plenty of similar threads to this one and I've explained before, Cyrus is just awfully annoying.
Other takes:
-Hatty - Cyrus' love slave and I hate her too
-Asoka - annoying, tech really fast, worst of that type of AI. Usually Asoka is worse than Gandhi, and worse than Mansa etc...
-Monty, he is the worst and most annoying AI warmongerer, the others just don't stack up as bad.
-Toku, is really annoying for never trading anything. He won't trade techs because he fears HE will become too advanced.
I generally find almost all of the BtS AI leaders actually acceptable, good all around or weak warmongerers but then I don't care. Sitting Bull is really annoying though, if nearby - if far away he's just a pain in another AI's side with constant espionage and defense-spam.