This list is based upon each character's quality. If there are any disagreements, please post in this thread.
Uber: Top Tier
Ike
Pit
Olimar
R.O.B.
Meta Knight
King Dedede
Overused:
Fox
Link
Zelda/Sheik
Toon Link
Snake
Bowser
Marth
Yoshi
Borderline:
Wolf
Mario
Luigi
Ice Climbers
Lucas
Samus
Lucario
Pokémon Trainer
Underused:
Sonic
Falco
Jigglypuff
Ness
Captain Falcon
Pikachu
Wario
Diddy Kong
Not Used:
Mr. Game & Watch
Ganondorf
Kirby
Peach
Donkey Kong
Uber: Top Tier
Ike
Pit
Olimar
R.O.B.
Meta Knight
King Dedede
Overused:
Fox
Link
Zelda/Sheik
Toon Link
Snake
Bowser
Marth
Yoshi
Borderline:
Wolf
Mario
Luigi
Ice Climbers
Lucas
Samus
Lucario
Pokémon Trainer
Underused:
Sonic
Falco
Jigglypuff
Ness
Captain Falcon
Pikachu
Wario
Diddy Kong
Not Used:
Mr. Game & Watch
Ganondorf
Kirby
Peach
Donkey Kong