I voted Sark because he is the most ruthless of them all. Remeber the mission with Syd trying to get the antidote? Sloane also is evil, but lots of the characters have some goodness in them.
I have to agree with the majority here, Sloane is the most evil of them all . . . perhaps he is more worse off after Emily died . . . I guess we'll have to wait for Season 3 to see exactly how the story unfolds or unfolded after the Telling.