Man best is so hard because most of the great Mario games are 2-D.
Best: I feel the same dilemma from which Bird Person suffers, except I'm torn between SMW, Yoshi's Island and SMB3. I'd have to go and say SMW, just because it's so much more in depth and harder to complete and what not, but SMB3 and Yoshi's island are both AWESOME.
Worst: Does Super Paper Mario count at all? If so, it's my #1 vote. If SPM doesn't apply for this poll, I think I'd have to say SMB2j. While it was fun, it did nothing new except make things harder. It doesn't seem as much as a standalone game as it does an expansion of the first SMB. An unnecessarily difficult expansion at that. Keep in mind though, it's still an alright game. If SMB2j isn't acceptable, then I'd have to say NSMB--although it's still good. It's just kind of a letdown, given that it has no awesome power-ups outside of the classic Mushroom and Fire Flower. I know it's supposed to be New Super Mario Bros., but would it have killed them to include the tanooki feather? NSMW would have been even better because there would be Yoshi riding.