My best switches between Peach and Zelda. Some days I'll be great with one and suck with the other, and then it changes. Back and forth, back and forth. My worst character, though, would probably be Sonic. I don't like the way he handles at all.
I rarely lose as Luigi, so I suppose he's my best. I do OK as Link too I guess. My worst has to be Sonic, it's not that I'm bad as him, but I lose a lot of games.