It's Sir Michael Caine's opinion that Heath Ledger beat the odds and topped Jack Nicholson's Joker from Batman (1989):
"Jack was like a clown figure, benign but wicked, maybe a killer old uncle. He could be funny and make you laugh. Heath's gone in a completely different direction to Jack, he's like a really scary psychopath. He's a lovely guy and his Joker is going to be a hell of a revelation in this picture."
Caine bases this belief on a scene where the Joker pays a visit to Bruce Wayne's penthouse. He'd never met Ledger before, so when Ledger arrived and performed he gave Caine such a fright he forgot his lines.
Heath Ledger VS Jack Nicholson