Mario is my best, and probably will be my best for SSBB. Hey, if you can defete Giga Bowser and Ganondorf in Event Match 51 with anyone besides Jigglypuff, you'd consiter them your best. Weird, because before SSBM came out, I always consitered Yoshi to be my best. eh.
Second place would go to Mewtwo. His play style may be challanging to some, but if you keep playing as him, he'll be cake.
However, if you keep playing as a character, you'll learn his moves with ease. Link is a character I used to avoid because he was weighted down more in SSBM than in the original. I eventually got over his weighted body and became formatable with him. Roy is another character who I'm good with. But yeah, who ISN'T gifted with Roy?
I can basicly use them all (I beat my brother with Pichu in a stamina match XD), though I do have some problems with Samus. I generally avoid her.