Solid Snake and Sonic the Hedgehog (or, at least Brawl does)
But really, the fighting system makes it extremely fun. Proof of this is Smash Bros.'s popularity. People don't play the game to see their favorite characters. They've got plenty of other games in which these characters appear. They play it because the unorthodox fighting system is awesome.
That, and the wide variety of things to do. In addition to just smashing the hell out of the computer or friends in the main game, there is also a pretty long and fun story mode (Subspace Emissary), Target Breaking courses, a Home Run contest, endurance fights, boss rush, etc.