Meta Knight is easily the best, but Wolf, Mr. Game and Watch, and Pikachu are good as well.
Elfnote wrote:
A useful trick for if you're trying to unlock stuff, if you only have to defeat a single AI controlled opponent, and they only have one life left, make sure you're playing as kirby. Suck the other player up and walk off the edge, it'll consider them dead before you and you wont be penalized as a result. This works in both Melee and Brawl.
You can use Kirby to spit people out underneath the stage, and then fly back up to make sure you don't die. I've annoyed people by getting repeated kills on them in that manner. Still, it doesn't always work—if your opponent knows what they're doing, they can get out of Kirby's mouth before they're too low to recover.
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WAR IS PEACE
FREEDOM IS SLAVERY
IGNORANCE IS STRENGTH