I've had a little goal to try to collect one game to represent each of the 86 characters on the Ultimate roster. I'll start here, and I may follow up later. My general criteria:
1) Physical strongly preferred. Digital is next, and NSO is in a distant third.
2) One game can represent multiple characters (i.e. Ken and Ryu, Mario and Bowser). But I will try to choose one game per character.
3) The character is ideally the main character or villain. Okay if they're just playable (ex. Daisy in Super Mario Wonder).
4) Ideally, the game is actually good.
5) It's preferable it's a new game, but re releases it collections will do.
I'll give my thoughts in order for now:
1) Mario - Obviously tons to choose from. Super Mario Odyssey, Super Mario Wonder, 3D All Stars, NSMB Wii U, etc. For Mario, I figure I'll pick Super Mario Maker 2 - a fun game which has the unique spin of community involvement in designing and sharing levels. I haven't played all that much of it, but I enjoyed what I played.
2) Donkey Kong - the best option here is DKC Tropical Freeze. The port of a highly acclaimed Wii U game, it's interesting we did not get a new DK the whole switch generation. Fun game, and I look forward to the DKCR port.
3) Link - obviously Breath of the Wild here. Absolute classic game, and his ultimate iteration is based on BotW. TotK, and the SS and Link's Awakening remakes are there as well. Plenty of options.
4) Samus - Metroid Dread baby.
4e) Dark Samus - no great choices, but
technically in the secret ending to Metroid Prime Remastered.
5) Yoshi - Yoshi's Crafted World. A cute bit extremely mediocre game, but we're not left with any great options. I hear wooly world is better...
6) Kirby - between Kirby Star Allies and Kirtby and the Forgotten Land, I give it to Forgotten Land. Super fun, relaxing game which marked a very solid transition into 3D.
7) Fox - the first game where we may dredge into the NSO catalogue to pull out Star Fox 64. A shame Star Fox Zero was so panned it wasn't worth porting. Edit: post 2 reminded me of Star Link, which has Fox (and Falco) as DLC characters.
8) Pikachu - Let's Go Pikachu. Any Pokmon game would probably due, but this is the only one I've played on Switch... and I thought it was pretty good! Not amazing, but just enough of a mix up on gameplay to make the nostalgia trip a good time.