Best: Hmm.. Well, the 2 Mario and Luigi games both had a great mix of it (the Tutorials, when you are learning moves). I hate to jump on the band wagon, but I also want to vote for Mario Galaxy (the Toy Time theme is probably among the best remixes that I've heard). If I have to choose one, SMG.
Worst: Uh.. well.. I want to vote for SMW, because I also hate waiting so long to hear the mix, but I think I'll vote for Super Mario Kart's version, since it's been so long since we've heard it (that's a tradition I wish they would bring back, but that's a rant for another topic..)
I nominate Best First Level too.