Brawl is by far the funniest Smash game to play with friends, directly because Sakurai had a tantrum about nerds trying to play it competitively so he just didn't care about the balancing at all. It's perfectly crafted to piss off anyone who tries to take it seriously, but also make anyone laugh their head off if you play it casually; it feels a lot more like Mario Party in that way than the other ones. 4 players, all items high, Ice Climber Summit, let's go!
The character balancing is complete bonkers, many of the attacks are near useless but then they'll have one attack that is absurdly good, or the character can do almost anything whereas another is total shit, and that just makes it so much funnier. The later games are nowhere near as absurd casually, directly because there's character balancing.
The items are really fun too, maybe excluding Super Spicy Curry, and the Smash Ball which appears faaar too often. I swear the game has some code that spawns an explosive once you start charging a Smash attack lol. Many of the items added in the later games are extremely annoying, stuff that hits you over and over and over again, or stuff like the Drill Arm and Gust Bellows that drag you all the way off the screen, or stuff that just ruins your controls like the Super Leaf, Jetpack, or that Earthbound mushroom.
Oh, and the Subspace Emissary story mode and Stage Builder was cool. Just a great game.