Gah! I can't choose one! I usually use Pichu, Pikachu, Kirby, Zelda, or Falco when I play. Zelda and Falco are good in the sense that they can reflet other people's attacks with B for Zelda or down B with Falco (and Fox seems too fast for my liking, as in seems), and Kirby can suck things up and get his opponent's B special power, how great is that!? And Pichu, though it can kill itself, is really good if you know how to use it (smaller=you can just crouch down and not get hurt, unlike Pikachu), and its side B move is better than Pika's when I played it. But Pika's good because, unlike Pichu, it doesn't hurt itself when it uses a special move.
And I occasionally, as in rarely-ocassionally, use Mewtwo, for the sake of annoying others who think Mewtwo is really cheap.
For now, I think I'm going to vote Pichu, because I think I generally use it more than the other players.