Marth: Hard to compete with the beast that was Melee Marth. He is different in Brawl; not better or worse.
DK: I believe he got one of, if not the, biggest buff from Melee. His back air is absolutely incredible; possible the only move he needs.
Zelda: Much stronger than Melee's version of Zelda. Can compete with just about anyone, especially in the camping scene with her Over-B and B move.
CF: Oh, mine sweet Douglas J. Falcon. How the mighty have fallen. In my opinion, was the most fun, balanced, and skill-based character in Melee. A proper use could turn CF into a killing machine. Now, with the new game mechanics and a severely nerfed air game, The Captain is but a remnant of his once glorious self.
Link and Samus: Tied for worst worst returning characters. For how powerful swords and ranged abilities are in Brawl, Link just gives both swords and range a bad name.
Mario: Some say he is worse because of the lack of combos. I actually think he is better, and can indeed still combo. His D-air is very good, and the rest of his moves constitute a solid repertoire.
Luigi: Although he lacks the wavedashing supremacy from melee, he is still a strong character, mostly because of the lack of any lag after any of his air moves. His Down-B is also a solid approach method.
Pikachu: I didn't think he was very good in Melee, and I do think he is great in Brawl, so that accounts for a significant buff. Camps well, and has a strong, unique close game.
GW: Probably runner up for best returner, GW recieved substantial power over his former self. Screw the parachute; fishbowl is a great move.
Kirby: Another honorable mention for most buffed, Kirby now presents a threat to most other characters.
That's it as far as my experience goes.