Because Robin Williams is a good actor, he would have done a good job with the Joker. I also concur with kurruption on the Jim Carey bit, though I don't really know the flambocancy of the Riddler from the comics.
Paul Bettany is still my choice. I agree with the idea of choosing a younger actor. Though I think Robin Williams is great, he is too old for where the Joker should be right now, in my opinion.
It could have been interesting though to see Carrell try and play an evil character. Though the Joker is humerous, in the end he is evil, as he should be.