I agree with Carrey if he can restrain himself. He has the build and the smile for it (remember The Mask? He looked a lot like The Joker). It also depends on what kind of Joker they go with. To me, the most successful Joker was that portrayed in The Dark Knight Returns followed by his portrayal in A Death in the Family. In both cases, there was none of that "let's find the humanity in the arch-villain" stuff (not a knock against The Killing Joke; I just disagree with the need to humanize every villain). In TDKR one of the psychiatrists says that "there's something supernatural about that one," one of the greatest quotes in the graphic novel. The killing spree he went on and the complete lack of reason for it (other than to taunt The Batman) was perfect, as was the lack of his constantly smiling (when he did, it was truly ghastly). In ADitF, he was a bit more theatric, but again, he was truly evil. The parting "Heh heh heh" before his helicopter crashes with him in it was superb and chilling.
I suspect, however, that they will go with a Killing Joke Joker, which to me robs the character of his essential otherness. We already have Two-Face as the study for humanity in the villain, we don't need the same for The Joker, who should represent everything against which The Batman stands opposed.
If they go with the former, a truly evil Joker, I suspect it'll be a relatively unknown actor. If they go with the latter, it'll be someone we know and possibly even Carrey.
