Project M is the best.

Though if we're not counting that, melee is the best because it's so fast and competitive. Sm4sh ranks second to me

The very best. Nuff said.

..Well, that and the removal of the tripping mechanic. It was beyond annoying. It defined the outcome of most matches more often than anything else.

And last but not least: Captain Falcon. Since his downfall in Brawl he's back to being a respectable fighter that is not just a cool-looking punching bag.

Smash 4 is my second favorite behind Melee, even if I suck at it :

Melee > 64 > Brawl > Smash 4

Based purely on gameplay, Brawl and Smash 4 would switch place. They both share the same flaws though (weak combo potential while overly rewarding defensive playstyles, leaving little room for exciting gameplay). The reason I put Brawl ahead of Smash 4 anyway is because Brawl has better music, a real tournament mode and an actual adventure mode, making in more enjoyable on a casual level at least.

Additionally, if 64 had more characters and stages, I'd consider ranking it above Melee. As it stands though Melee has by far the greatest variety and potential in terms of exciting gameplay.