Can't really pick one...
Melee - Zelda/Sheik was a brilliant way to handle that character. Ice Climbers were a unique and interesting approach that was exciting. Bowser pushed the boundary of just the scale and weight characters could have.
Brawl - Pit and Metaknight were just awesome. The flight mechanic added a new dimension to consider. Wario was a charming yet obvious inclusion. Zero Suit Samus added an interesting dynamic to a pre-established character. Also Snake. Because Snake.
U/3DS - Wii Fit Trainer is Smash Bros. character selection at its best and most clever. MegaMan compensating for all the different weapons is an interesting dynamic to balance. Villager is just silly. Rosalina is another unique and creative way to introduce a new character. Little Mac seems obvious but I'm not certain as to how he'll fare.
Melee - Zelda/Sheik was a brilliant way to handle that character. Ice Climbers were a unique and interesting approach that was exciting. Bowser pushed the boundary of just the scale and weight characters could have.
Brawl - Pit and Metaknight were just awesome. The flight mechanic added a new dimension to consider. Wario was a charming yet obvious inclusion. Zero Suit Samus added an interesting dynamic to a pre-established character. Also Snake. Because Snake.
U/3DS - Wii Fit Trainer is Smash Bros. character selection at its best and most clever. MegaMan compensating for all the different weapons is an interesting dynamic to balance. Villager is just silly. Rosalina is another unique and creative way to introduce a new character. Little Mac seems obvious but I'm not certain as to how he'll fare.