I still play Brawl, Smash 64, and Melee to this day even while Smash 4 is still out. I love all the Smash games equally. I love how each game is completely unique from each other and has its own strengths:
Smash 64: For newcomers learning how to play
-The most simplest Smash and by far the best introduction to Smash Bros for newcomers wanting to get in the series. Not all mechanics from Melee and onward are implemented, making it easier to learn. This game is for the complete newcomer who finds Smash confusing and doesn't know how to start. Once they mastered 64 and learned the rules of Smash, they can move on with confidence to other installments and master those games.
-A game all about the combos. With the massive hitstun and the extremely satisfying sound you hear whenever you land a hit, combo-enthusiasts are sure to have a blast.
-The most chaotic 4-player free-for-all experience next to Smash 4. Especially with items on.
-Has the best items in the entire series. It's actually really fun to turn items on in this game and they always improve the experience rather than hinder it.
But I will say that Melee's 4-player is BOOOOOOOOOORRRRRRIIIIIIIIIING.