A future thread should have us vote on the features of remakes. SMA4's E-Reader levels, SM64DS's added characters, etc..
Best: Hmm.. If we're going with platformers, I'd vote New SMB. If RPG's, M&L: Superstar Saga. If I have to pick one, M&L. Honorable mention to Mario Kart: Super Circuit for being the first handheld MK game.
Worst: Mario VS DK. It looks great and had it's moments, but it doesn't play very well. I don't know how many times I lost a level because of near impossible time limits. Honorable worst to Mario Party Advance: only four playable characters and a game full of minigames where you play by yourself (against the timer, or the Baby Bowsers) is not fun. They could have called it Mario Ware, perhaps..