It's pretty close between Brawl (didn't like the slower more floaty physics) and Wii U (basically a weaker, more stripped down version of the current game). Never played the 3DS version but I'd imagine it'd be in the running for most irrelivent as well.
Smash 64 has the nostalgia charm, and there's a certain appeal to the simplicity
Melee was the perfect balance of simplicity and complexity, plus the mechanics are tight and solid
Smash for Switch is just chock full of content, it's the ultimate package with the slickest graphics, and it's cool that you can play a full blown Smash game via home console and handheld.