Melee > PM > Wii U > 64 > Brawl > 3DS
Melee is the most balanced out of the games that are set in stone (games with no patches), has the highest skill cap, is the fastest, in my opinion is fun to watch, and is a great game to play casually or competitively. PM is the same, but can be glitchy (try entering Tournament mode), some characters are unbalanced, has easier button inputs because Brawl, and the characters are not in a set teir list and are constantly being changed. Wii U is fun, but I still don't like the new edgeguarding. I like all of the new ideas like custom movesets and 8-player battles, though. 64 is very fun, we wouldn't have the rest of the series if it were not for this entry, and is very simple in design (more so than the others). Brawl wasn't much good competitively, but it had much more content than Melee, a story mode (rather than the Adventure mode which was only exciting the first few times), stickers, more trophies, and extra music. The gameplay really pulled it behind, though. I don't like the 3DS's controls as much as a GC controller or N64 controller, there will always be multiplayer lag (even if it isn't noticeable, there will still be the excuse), the not-so-ledgeguarding mechanic in Wii U coupled with the bad controller makes for deaths that shouldn't have happened, and it is the Smash game where I ran out of stuff to do the fastest.
Also, Melee was my first console game for a Nintendo system, as I got it on my sixth birthday in 2003 with my GameCube. Because of that, I played Melee way too much and it is now my favorite game of all time. And now I am in the competitive scene and go to tournaments playing my favorite game of all time with people that love the game as much as I do.