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!
The Nintendo eShop rating Thread: http://gamrconnect.vgchartz.com/thread.php?id=237454
The Steam/GOG key gifting thread: https://gamrconnect.vgchartz.com/thread/242024/the-steamgog-key-gifting-thread/1/