Brawl. After playing it for so long and going back to Melee (erasing all data as tradition), I found the game to be unplayable. Being immobilized after so many different actions, incredibly bad recovery, Gravity is twice as fast to the point of being unfair and unrealistic, weapons like the hammer and bat are way too dangerous as well as attacks such as a longer ranged falcon punch, and four characters in a match is very unplayable as they literally find any possible chance to gang up on you, whether it's teams or not (you can literally go from 0 to 80% damage in five seconds against three computers, which is why this game deserves the title of "Melee" which usually simulates one on one matches, where Brawl means more than two fighters).
Melee may be really great because of nostalgic effect for a bit, but it makes you want to turn the damn game off after so much frustration. In short, it was good for its time, but this is probably one of the few cases in gaming where you must upgrade to the newer installment, or get the oldest and work your way up.
Brawl is the greatest game ever made on so many levels.