I'd probably say Smash Wii U. The sheer variety of characters, the controller options, amiibo functionality, all that fun stuff... I'm always blown away by how much they packed into that game.
I grew up with Melee, played it when it first came out, but there're so much more to do in Smash 4 so even tho there's a lot of great memories with Melee. I've to say Smash 4
Yeah, if Smash Switch has a lot of the variety and playability options of the Wii U Smash, with a Melee esque Adventure Mode... that's the kinda game that will remain in people's Switches for ages to come.