I can play as any character, really, but my best character is Toon Link. I LOVE the sword-weidlers in the game, (Link, Toon Link, Marth, Ike, etc.), but Toon Link is the best for me, simply because he's fast, light, and more of an all-around character than themed on a single advantage. Plus, I've beat-up 3 level 9 Ganondorfs with my boomerang in slow motion. It was awesome.
My worst would need to be a tie between Ganondorf, Bowser, King Dedede, Ness, Lucas, ROB, and Mr. Game and Watch. Honestly, I don't see how anyone can play as them. I'm still developing a basic system for them, and none seem to work.