It'd have to be Fred Phelps at the moment, but only if I can just appear from nowhere thru time, dressed as Jesus, and tell him that God is real, and I've been sent to end him because of his constant misuse of God's name in the pursuit of hatred, then snap his neck like a twig.
Then just leave a note on the body saying 'Jesus Saves (gays too)'. Oh and pull his pants down and surround him with gay porn and a bottle of lube, then call the press, opening a window and telling them. I imagine so many people would just believe it that they wouldn't even get as far as noticing a broken neck.
There's a lot of people I could have said , like Hitler, Kim Jong Il, Simon Cowell, Piers Morgan, but I fear a number 2 would just pop up to fill the seat now vacant.
Destroying the leader of the WBC 'might' shake em enough, I don't know, but it's worth a try, tho to be honest, when I see him, apart from thinking of flying Phantasm balls, I figure he looks so frail, popping a balloon near him when he wasn't ready might just finish him off.