Outer wilds is ridiculously good, it's such a crazy unique experience. But you have to go in without spoilers, it's not nearly as fun if you know where things are.
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
Upvoted, we had so much fun with PlateUp! I think we also have more playing time for PlateUp than for Overcooked 1 & 2 combined (both completed). I love the roguelike element in it and building our own restaurant.
There is this little hidden gem called The Witcher 3.
Ok, real answer: To the Moon and Doki Doki Literature Club are two indies that really left a mark on me, specially the former. Both games are fantastic.
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...
The original Cave Story is truly a piece of art. I could not believe back then how good this game is that was created by a single person. He did everything. In an Interview Pixel(?) said that he learned composing music while creating the game. He had no knowledge in that field and just tried and stocked to what sounded nice to him. He created one of the most memorable soundtracks I ever heard! What an unbelievable talented and creative person. So inspiring!
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 play indie stuff. It feels more like a labor of love than big, corporate AAA titles. That said, the only two must haves, Imo, are Omori and Dwarf Fortress. Neither game is perfect, but they are both incredible, unique experiences. With Dwarf Fortress, enter with plenty of prior knowledge, but with Omori, play it blind. Very important for both games.