No Mehmed in poll?
Gilgamesh is pretty bad. He gets big borders which clash hard with my own, he's insanely hard to dislodge by force, and while he's not very aggressive, he's a real grouch who tends to refuse to actually help me in any way.
Catherine's about as bad due to sheer unpredictableness. She's all nice and helpful until she decides someone has to die, at which point it doesn't matter at all who her friends are; the weakest looking target gets picked on.
Shaka is like a big ugly zit that's painful to pop, but will never trouble me again once I suck it up and get it over with.
Gilgamesh is like a festering ulcer that will never, ever go away.
Catherine is like a cancerous tumor that springs up out of nowhere after years of not having it and never suspecting that I'll get one.
I have to vote for Joao, because I always expect war from Shaka, Monty, Catherine et. al. But Joao has a special knack for pouncing when my armies are on the other border, my economy is suffering and other civs happen to have the -1 "you declared war on my friend" modifier with me.
In short, Joao is the best at stabbing me when I don't expect it/ can't do anything about it, and thus, a tip of the hat to him!
Easier to say who are not the backstabbers. Anyways, the Jerks awards goes to:
Catherine - my vote
Joao
Peter
Alexander
Shaka
Napoleon
Montezuma
Justinian
Louis XIV
The Khans
Dont trust in a russian, french or mongol in game.