I'd say Crazy Hand simply because it's a harder version of the master hand, kind of like a true boss. Whenever I do well enough to warrant Crazy Hand's appearance, I always feel like it's "game time" and go serious mode for the whole fight.
..too bad I won't be able to get the next SSB game. :\
Both hands would be my vote. I mean they're like the creators of this universe (remembering how Master Hand set up the fights in the first game's opening) and they're bad ass when put together. So yeah, fear them hands