Super Smash bros started it all
Melee brought it to the competitive scene
Brawl introduced the story mode
Wii U brought 3rd party characters and challenges
3DS made it portable
Ultimate is all of the above combined into one game. All of them were necessary stepstones to what Ultimate was to become.
However, if we talk about replaying any of those today, then I'd say Brawl, simply because it has a story mode which the others lack. Playing competitively is ultimate only now, so casually I'd prefer a story to randomness.
Oh, and about future Ultimate updates in the season pass: Bring back Home run and target smash!
Brawl introduced 3rd party characters, Solid Snake and Sonic first appeared in Brawl.
Yeah, but that was more of a test, Wii U/3DS very vastly expanded on this by adding tons of third party characters assist trophies and Mii costumes.
Besides, my listing is more about what defines each of those parts; Snake and Sonic are not the thing that sets Brawl apart, it's it's Story mode that does.