I like Monty, actually. He's such a great pet dog, but always so, so willing to bite the hand that feeds him. He's kinda fun.
I think the AI leader I hate the most is Saladin. In my games he's always a tech maniac who won't trade to you without the same religion.
But, to be totally honest, I usually hate the IA who is giving me the biggest run for my money at any given moment. =)
I'm an equal-opportunity despiser.