Tim Burton, absolutely.

I think he has not only a stunning vision of fantasy and myth, but also a lovely twisted sense of humour. I'd love to see what he'd do with the Hobbit. Of the others, well, Guillermo del Toro would be interesting as well.
PJ, on the other hand, would do well to leave the Hobbit for someone else. I'm quite bored of his massive, effect-oriented style of picturing Middle-Earth and I'd love to see another director's vision as well. Besides, I wouldn't like ME becoming known only by PJ's versions.