Best: While I also liked the 2D Bowser levels in PM2, my favorite form of Bowser is.. the cartoon form! Ok, since the Super Show/SMW isn't a game, I seriously vote for the console Mario Parties (1-3 on the N64, 4-7 on the GC, and 8 I guess..) They show him for what he is: a Koopa King with magic powers (remember: the story for SMB said that he turned the citizens of the MK into blocks, bricks, and weeds) When he shows up in the MP games, he uses his magic to.. change the rules of the game. Ok, so that's a bit different than taking over the world, but an evil magic user takes what he can.
Also, I really like his character in SMG, so I guess that's one vote for MP (pick one) and one for SMG.
I'll come back and do my Worst later..
Worst: I don't know.. there were some that were better than others, but I don't particularly think there is a worst one..
Nominee: Non-Mario playable character.