S: Fox, Falco, Jigglypuff, Sheik
A: Marth, Peach, Captain Falcon
B: Ice Climbers
C: Dr. Mario, Ganondorf, Samus
D: Pikachu, Mario, Luigi, Donkey Kong
E: Link, Young Link
F: Zelda, Roy, Mewtwo, Yoshi, Mr. Game & Watch
G: Ness, Bowser, Kirby, Pichu
The 8 top tiers- everyone ICs and up
The 6 super viables (characters you can make your main without any real need of a secondary) - everyone Luigi and up
The 7 "specialists" (characters who have major flaws overall but are still great at certain matchups; ex: Y.L v. Puff, Link v. Marth/Samus, Zelda v. ICs, Roy v. Falcon, etc) - Yoshi and up
The 5 at the bottom (characters that struggle, and struggle hard, at just about every matchup) - Kirby, Pichu, Bowser, Ness, and G&W
Not sure about Brawl right now, but when I played, MK dominated, and then it was Falco, Wario, Snake, ICs, Diddy pretty even.