Smash games:
Favorite (casually): Brawl. Jam-packed with single player and local multiplayer content. Best item selection in terms of balance. Fun wacky stages. An incredible collect-a-thon and a lot of love and sweat went into making this game for the average Nintendo fan. Very weak online aside, casually this game rocks.
Favorite (competitively): Melee. fast, technical, and very rewarding offensively. A sharp game that tests your mental strength and reflexes.
Least favorite (casually): Smash 64. I mean, the multiplayer is fun to this day, but otherwise you can see everything this game has to offer on a casual scale in one full day.
Least favorite (competitively): Brawl. Due to tripping, planking, safe shields, wack balancing, and chain grabs. Smash 4's defensive game can be ridiculous but thank heavens they got rid of most of Brawl's flaws. Not surprising that Brawl is weak competitvely when Sakurai intentionally tried to nullify that side.