Washington is nice to see since he makes a pretty reliable ally for resources, although he tends to get steamrolled by the other AIs.
My absolute favorite may just be Genghis Khan of all people. While he tends to be aggressive, he can also be strangely peaceful for long stretches of the game, especially when you manage to get on his good side. Even when I play against him, it's hard to tell exactly how he'll act in a given game until he begins to make his first major moves, which can be interesting. But mostly, I just find his cheerful greetings of "hey, sausage man" endearing. *shrugs*
Also I'll have to second Smallfish's mention of Monty. Yes he was on my most-hated list, but he's also one of my most loved. As annoying as he can be, he just looks like he's having so much fun. At least you know he's enjoying himself when he plays against you.