Smash 64: Hardest- Ness, he's a tricky one.
Easiest: I'd say Pikachu, mad range, easy recovery.
Melee: Hardest: Yoshi or Fox, to play at a high level. Fox is crazy to master, but not too hard to win with, whereas Yoshi is crazy hard to master AND win with.
Easiest: Shiek or Marth, Shiek is really easy to combo with, you barely even need L-cancelling and Wavedashing to be effective. Marth just has that mad range, you'll be good so long as you use it.
Brawl: Hardest IMO is Shiek or Yoshi. Shieks DACUS is craaaaze, Yoshis Dragonic reverse is TOOOO craze. Shiek has a hard time killing and Yoshi gets ****ed right up by anybody who can stay in the air for a long time (half the cast)
Easiest: MK. Doesn't have any really difficult techs, has no problem killing, attacking, or approaching. He doesn't even have a terrible match-up, 60-40 at worst. ****ing MK....