Jake Gyllenhaal
I know you all may want to call bullshit and slap me in the face for this one, but hear me out before you do that. Gyllenhaal is a damn fine actor, and not above playing dark or twisted characters. Just go back to his early career and recall the majesty of Donnie Darko. He has some looks and moments in that movie where (even though he is not a “bad guy” in the film) he is creepy as hell. That little grin he wears is quite memorable, too. So imagine THAT side of him, but only as an insane madman dressed as a clown. It could totally work.
Also, don’t try to pull that “he is too good looking” shit, because a ton of us tried to pull that card when we heard Ledger was playing the Joker, and we ended up eating our words. Plus, him and Ledger had a thing in Brokeback Mountain, so that would make the connection even more surreal.