People have already given a lot of good recommendations but as far as Nintendo exclusives go I'd also recommend Bayonetta 2 and Wonderful 101: Remastered.
I'd start from those upgraded and from games that were already running great:
Super Mario Odyssey
Zelda BOTW
Zelda TOTK (but take a good break from BOTW first)
Metroid Dread
Metroid Prime Remastered
Smash Ultimate
As much as I'd love to recommend the best JRPG franchise ever (Xenoblade) it's not upgraded yet so I'd wait. Same for any other game.
Cartridge Titles:
Mario Odyssey
Mario Wonder
Mario 3D World + Bowser's Fury
Zelda Tears of the Kingdom
Yoshi's Crafted World
Kirby Superstar Saga
Donkey Kong Tropical Freeze
Download Titles:
Sega Ages Phantasy Star
Stardew Valley
Collection of Mana
Nintendo Online NES, SNES, and Gameboy - dozens of good games on here.
Most carts can be had for $35 used right now, but that won't last forever. Third party online titles routinely go on sale for $5-$10.
So GAF, I just bought a Switch 2 and I don't own an original Switch, this thread is for you beautiful Switch connoisseurs to make recommendations of the best Switch games to play. They can be the obvious classics, hidden gems or just games you like.
Mario + Rabbids. Probably the most pleasantly surprised I've ever been from a video game. Ubisoft done Nintendo and themselves proud. A great game which was also stealth patched to 60fps.
If you enjoy it the sequel is a larger budget version set in space but I have to be honest I have a major soft spot for the original despite the increased scope and budget of the sequel. Both great casual RTS games though and both extremely cheap nowadays.
Astral Chain
Xenoblade 1, 2, 3, X, and their assorted DLCs (the DLCs for 2 & 3 are both the length of normal games)
Fire Emblem Three Houses & Engage
Triangle Strategy
Live a Live
And, something new that I haven't seen mentioned yet:
It's a Visual Novel, and I'm not really into visual novels beyond using them to learn Japanese, but it's damn good. Well worth the price and you can just chill and read it randomly until you complete it.