Subnautica. One of the best experiences I’ve had in gaming.

Outer Wilds. One of the most creative and thought-provoking games I’ve played. Definitely go into this one knowing as little as possible about it.

Hollow Knight also deserves a spot here, but personally I preferred Salt and Sanctuary.

FTL, Into the Breach, Celeste, Hollow Knight, Banished

Valheim - especially if you fork out for a dedicated server. I was playing with family and friends across the Atlanic, so we’d log on the next day and find new bases and bridges built, etc.

I just wish you could modify and expand the boats!

Airships: Conquer the Skies - a great little 2D airship builder and combat game

Songs of Syx - a fun city builder even if combat is largely unfinished still

Not posted here yet: Hades. Though, all of Supergiant’s games are worth playing.

Oh disco elysium for sure. Fantastic crpg.

I would say Hades.
The gameplay loop, the art direction and the story are all very good.

Factorio with a friend. (Alone is still good)

Factorio has been my crack-torio for many years. Satisfactory may have better graphics, but I prefer the simplicity and depth of Factorio.

Gris. Gorgeous visuals and stress-free gameplay

Not incredibly long either

Not posted here yet: Hades. Though, all of Supergiant’s games are worth playing.

There are a lot I’d recommend, but I think the only must-play for me is Cave Story.

When I was growing up, Cave Story was like the paragon of games as art, and was a major inspiration behind me making games as a hobby and getting into software development professionally. Cave Story was made by a single talented auteur, not for profit, and released for free. And it was as good as Metroid and Mario, or maybe even better. It proved to everyone that such a thing was even possible.

I think the only indie game that could possibly compete with Cave Story for the title of “most influential” would be Minecraft. Though Minecraft should probably be disqualified from that title for having sold out to Microsoft as soon as it started to get big…

A ton of great suggestions here already. Let me add some more.

Dead Cells: amazing 2D action rogue lite with metroid Vania elements. A game that keeps getting updates faster than I can play them. And they are good.

Battle Brothers: hex based tactics and management game where you play a band of mercenaries in a fantasy world. It’s insanely deep and we’ll made.

I usually only play the large bombast blockbuster titles, but Actual Sunshine did leave me speechless.

Hades, though I don’t know if it’s a must-play for everyone. I like it a lot and is probably my most-played game on the Steam Deck.

Disco Elysium blew me away with the writing, absolutely a must-play.