It's funny, I just saw Batman on TV this morning and was thinking about who could play the Joker in a new movie. I thought that Jack Nicholson did a fine job (with the material he had to work with, at least), but he just didn't look the part -- the Joker's supposed to be well over six feet tall and built like a rail.
I liked Cesar Romero as the Joker. Weird thing to say, I know, since the '60s portrayal was waaaay different from the way he is now, but I think he captured the Clown Prince's unbridled mania pretty well. Makeup-covered moustache notwitstanding.
Mark Hamill will always be the voice of the Joker for me. Whenever I read a Batman comic, my mind's ear hears Hamill's voice coming out of the character's mouth. I would love it if he were to dub over the character's dialogue -- but I don't think that's gonna happen.
I think Adrien Brody would look good as the Joker, but I don't know if he could fly off the handle the way Mister J does on occasion. If it were the Dark Knight Returns version of the Joker, I would say Brody all the way -- I was pretty terrified by him in that story. But I don't think that the low-key manner he was presented was really typical of how he is in the mainstream comics. The thing about the Joker is that he can be pretty funny, in a "so freaky you need to laugh to keep from getting too disturbed" kinda way, and I don't think Brody's right for that.
As far as other Batman villains go... Jim Carrey could have been good as the Riddler had Batman Forever been a better movie. He can do creepy and keep it understated, but the director just didn't know how to handle him or the character.
One thing I would love to see is Robin Williams as Jervis Tetch, AKA the Mad Hatter. Only if he can do it really creepy-like, though, like in One Hour Photo.