My favorite goofy Joker moment is after he pays a visit to Kim Basinger and pops one in Bruce Wayne's chest, as he walks out the door, he stops, leans back and goes "Ah-phhhhhhhhhhbt!" before walking away.
What's going to be great about The Dark Knight is that Ledger's Joker is going to be so radically different to Nicholson's Joker that I won't be able to compare the two.
I'm sure a lot of other people will try, but it's no contest.
Nicholson's Joker is only rivaled by Gene Hackman's Lex Luthor on the list of Awesome Comic Book Movie Villains. Ian McKellen as Magneto comes close, but he's not quite at their level.