You've already got a bunch of great suggestions, but I'll throw my two cents in too. I think of these as indies or games that have a niche audience, but some of them probably have bigger teams involved.
Outer Wilds. It's a quiet, contemplative game about space exploration and seems unremarkable until you really start exploring and learning about the solar system you're in. If you play it, go in blind. IMO it's a perfect game.
Pentiment. You play as a young artist visiting a Renaissance-era hamlet to work on your masterpiece. The game is gorgeous and thoughtful about the decisions the player has to make when spending time and influencing the narrative. Stayed with me long after I finished.
Slay the Spire. Tightly-balanced deck builder rougelike. I've put untold hours into this one on maybe 3 different platforms?
Wildermyth. Hard to describe this one--it's like an interactive fantasy tale with characters that grow and evolve in ways you won't expect, and dynamically told so no two stories are quite the same. Pretty solid combat mechanics, too.
Undertale. Does Undertale count as relatively unknown? If so, Undertale.
Case of the Golden Idol. Very good mystery game with a cool, novel mechanic for solving each mystery. Don't let the aesthetic put you off.
Return of the Obra Dinn. Another incredible mystery game where you are determining what happened to the crew of a ship that sailed into port with no one aboard.
Papers Please. Simple mechanics that really make you FEEL like you're -Spider-Man- an oppressed civilian in a fascist/totalitarian government.
Disco Elysium. Another great mystery game, wherein part of the mystery is: what kind of person were you, and who are you now?
Tunic. It's like old-school Zelda but with more depth and some serious twists.
Stanley Parable. It's funny and fun.
Tchia. It's like Breath of the Wild if Hyrule was a real place and the game devs wanted you to love it as much as they do. Fun and charming in a big way.
Season. Hard to describe and I haven't finished it yet, but the opening stuck with me. It's about what we remember and what we choose to forget.
Hollow Knight. Does Dark Souls arguably better than Dark Souls. I was put off by the Dark Soulsness and the aesthetic, but once I got a few hours in I was fully hooked. Another perfect game.
Deep Rock Galactic. Great fun mining valuables and fighting alien bugs with friends.
13 Sentinels: Aegis Rim. A time-traveling anime story game with mecha combat.
The Forgotten City. A time loop game where you are sent back to ancient Roman time.
Nonary Games / Danganronpa. Both series are anime-inspired mystery games. If you want something that tries to be more cerebral and serious, Nonary is your ticket. If you want tongue-in-cheek violence, Danganronpa is the way to go.
The Witness. A masterpiece puzzle game built on an incredibly simple core design. One of my favorite all time "a ha" moments came from this game.
Citizen Sleeper. You are a synthetic being trying to survive on a space station and evade the megacorp that "owns" you. Your body is breaking down and you don't know anyone, so you have to take it one day at a time and do what you can to survive. Clever mechanics and a really well told narrative.
That ended up being a lot more than I intended to share, but if this convinces anyone to give any of these games a shot, I think they're in for a treat.
You've already got a bunch of great suggestions, but I'll throw my two cents in too. I think of these as indies or games that have a niche audience, but some of them probably have bigger teams involved.
That ended up being a lot more than I intended to share, but if this convinces anyone to give any of these games a shot, I think they're in for a treat.