The AI I hate most ingame is Montezuma.. No matter what civilization I play as, he will, without fail, declare war on me before I even find his territory. He's often declared war on me twice before I finish researching classical age techs. By the end-game situations, he's declared enough wars on me that I begin to think he just may not like me. Due to the fact that he tends to declare war on me almost immediately following a scout finding me (or vice versa), I have tended to become an isolationist civ.. I dread searching outside anymore.. I know he's out to get me..
On the other side, I hate Napoleon for his rapid expansion tendencies, and I hate Isabella for being a snobby girl who throws tantrums each time she doesn't get what she wants. I just dont find it reasonable for her to demand I give her my entire treasury and tech's just because her name starts with 'I', and ends in 'sabella'.. Unless there's a better reason that I ain't seeing.