Reaver gets my vote as well. He's actually worse in Fable 2.
Spoilers
He will kill-steal the final boss, the man who murdered your sister (a young child) and who you've been hunting for the entire game spanning decades, if you don't interrupt his monologue first. He does this because he finds him annoying, not because he has any real beef with him.
There's also a quest when you first try to get his help where he "tricks" you (it's very obvious, but writer fiat strikes again) into sacrificing your youth to uphold his deal for immortality with some evil fae-like beings (beings who seem to be connected to Jack of Blades, the first game's villain). He then betrays you to the final boss, apparently just because he's an asshole.
Oh, and he also kills a fan-favorite side character, one of the few people to show you and your sister kindness when you were destitute orphans living on the streets, because Reaver was annoyed that a photograph taken of himself needed to be developed before he could see it.
It's beyond enraging that you can never get back at him for any of these things. He's still around generations later in Fable 3, where he's a wealthy industrialist exploiting orphans. Of course.