I picked Sark. The boy's a prodigy at being all evil and, even though I adore him, he might as well be the devil himself for all the thought he gives to the sanctity of life and the suffering of others.
Next, is Anna Espinoza. I loved her evilness. Cool villain. I'm way over Sloane, though. I think his story arc is stick-a-fork-in-it done.