Theodora, Wu, Isabella, William, Harun, and Rameses rarely DoW on me, and if I want to provoke a way, it generally takes a lot of effort. Theodora is the least peaceful on this list, I'd say, but only because she seems to be bribed into wars easily (she almost always declares with someone more aggressive). Ramses tends to hate my guts, but he'll very rarely act on it, unless I become a runaway and start spamming wonders.
Khan, Washington, Boudicca, and Oba love to attack, but generally only early. All three of them can become good friends after the initial attack, and Mr. Khan is probably the best watchdog in the game, loyal and powerful enough to actually help. Washington is prone to backstabbing late.
Gustavus, Pacal, Elisabeth, and Haile tend to have an aggressive attitude toward certain people from the start, but only Elisabeth is sure to turn on everyone at some point. (Though everyone will hear "By the grace of God, your days are numbered!" with her in the game). She tends to bite off more than she can chew, however, and Pacal and Haile don't generally mount enough of an offensive to be dangerous. Gustavus gets a little cranky over warmongering, but he's not hard to get and maintain a friendship with.
Siam (not even going to try the name), Napoleon, Augustus, Cathy, Hiawatha, Darius, Alex, and Bismark will all hunt you down like a dog if you are in their way. All are also prone to becoming runaways, and to make it worse, Persia and Siam tend to be on everyone's good side until they declare war on them, making both powerful and hard to successfully counter (leave them one city, chain denouements are coming, just defend, and they'll come back for more later). Augustus is more competent than most with his units, Hiawatha generally has terrible land for trying to attack, and Alex WILL control several CS's. Avoid these like the plague if you want peace!
Monty is a cat if you are strong. He's always afraid early it seems, but he tends to spam units to pump up his point sticks number, though when he attacks, he's normally using hordes of low tech units. If you stay prepared for war, it's unlikely he'll ever pull the trigger.
Gandhi deserves special mention. He's peaceful if you are, but he tends to finish wars once started, and he can often take on the more aggressive civs early, making him dangerous. He'll also be friends with everyone sooner or later, so either staying on his good side or exerting more diplomatic pressure than him (it takes some time to master, but it is possible) is mandatory with him in the game, otherwise everyone will late you.
Pretty much everyone else is a wildcard. Dido can be hellbent on world domination or say she's your best bud and actually mean it. Suleiman is a force to be reckoned with, but he is also quite loyal unless he just happens to decide he wants to be a jerk that game.