Stardew Valley by a long shot.

Hollow Knight is up there too.

FTL and to a lesser extent their second game Into The Breach.

RNG heavy strategy games with lots of micromanaging.

No other game has had the impact on my way of thinking more than Outer Wilds.

Slay the Spire. I've probably put more hours into this game than any other in my life.

From there, I guess all the usual picks. Hades, Hollow Knight, Braid, Fez, Dead Cells, Celeste

FTL - Faster Than Light

Dead Cells

Factorio. Help I got 4k hours and I still get cravings.

Honorable mentions.

Rimworld, Dyson sphere program, Minecraft (before it became microcrap)

Minecraft. I know it's a big company now or whatever, but back when it was just Notch, it was still completely captivating.

Also, Stardew Valley!

Dead Cells

Terraria

Hollow Knight

Risk of Rain (both 1 and 2)

Hades

Factorio

Balatro (my newest addiction)

Uh... I swear I wanted to contribute just 2 or 3 games, but as I wrote, I kept remembering one gem after another... oh well... :)

Outer Wilds - So hard to describe, it's an exploration game, but what you're exploring is a star system going supernova, in a wooden spaceship no less. And a strange way of (not) time travel is also involved, which could be the root of the whole game loop.

Axiom Verge - A platformer that is such a labor of love that it hits just the perfect mix of approachability, exploration, story development and that "huh?" factor where right until the end you're not sure what your abilities actually mean - i.e. if you could glitch through walls in the real world, would that imply the real world is a simulation?

Stardew Valley - A somehow utterly satisfying farming simulator in the style of the first Harvest Moon games. Such a nice getaway game - it begins with your avatar quitting their office job and moving to a farm inherited from their grandfather. No taxes, no boss, no stress, just rise with the sun, plant, water, harvest and fix. Change your rhythm with the weather and the seasons, investigate charming little mysteries of a beautiful place.

Broforce - Another platformer, this one a bit more brutal. Far over the top 80s action heroes bring freedom to the world, but whether you play as Robocop, Schwarzenegger, McGyver, Snake Plissken, Ripley or another 50 heroes is almost random and each hero has completely different weapons and skills. Destructible environment and even a large Xenomorph outbreak (how the heck did they get the license or grant?).

Protolife - This one uses such a madly simple recipe for complex gameplay. Seen top-down, you're a robotic loader than can put down dots. That's all. But certain arrangements of dots are guns, long range guns, flame throwers, area denial, missile silos, barriers and so on. You're attacked by insect-like creatures, but instead of building tanks, you have to attack via well-placed guns slowly pushing the swarming enemies back.

Alien Shooter 2 Reloaded - Simple top-down shooter where you're the lone soldier seeking to contain an alien outbreak. Goes for the time-honed recipe of character stat upgrades (speed, health, accuracy) and purchasing weapons and weapon upgrades. The interesting part is the insane hordes you're up against and that all the corpses stay. It's not unusual for entire corridors to turn into flesh hallways of blood and carapaces.

Moons of Madness - I hope this is actually indie, the graphics are near AAA level. It's 50% walking simulator, 50% cosmic horror, set on Mars. You're an astronaut doing maintenance on an outpost, but rather than go for the "freaky alien attack" recipe, reality itself seems to be somehow bending. Cthulhu, is that you?

Lumencraft - Top-down game. You begin as a miner in an underground base. Something really bad happened to humanity and now you're digging underground for metal and for "lumen." To feed the reactor that keeps humanity alive, you have to meet harvesting goals and dig tunnels, but various enemies attack in waves, so you have to spend part of your resources on fortifications and turrets and avoid opening up too many avenues into your bases.

Carrion - 2D platformer-ish. In a secret place, scientists are holding a horrific, tentacled bioweapon locked away, but it escapes. Twist: you are the tentacled bioweapon, slithering through pipes, circumventing security systems and trying to escape from the lab.

Nuclear Blaze - 2D platformer. You're a fireman sent to contain a fire the broke out in some kind of installation in a forest. But one building has a shaft that leads deep underground where a high-end containment facility is suffering a failure. Takes place in the "SCP" universe and your only tool is a fire hose. Extremely fun trying to extinguish fires in a way where they won't spread again.

Mothergunship - This is a first-person shooter where you're bording and destroying (from the inside out) an army of AI space ships. But instead of a traditional gun, you have gun parts you can stick together. How about a triple rocket launcher with two shotguns in the middle? Or a shield generating laser with a sawblade attache to it, and maybe two shotguns just to be sure? It doesn't grow old with new weapon parts being introduced right until the very end.

Space Run - 2D base building. You're a mercenary cargo pilot fending off space pirates. But you don't do it by controlling a turret, instead, your spaceship is a building surface and you have to build the right kind of engines, turrets, shields and power generators (in mid-flight no less) to be able to shoot down incoming rocks and pirate ships. Extremely well balanced and fun.

Creeper World - 3D real-time strategy. But your enemy is not actually present on the map, you're just fighting a simulation of liquid, a gooey slime that pours out of several spots. You have to keep shooting, bombarding and containing the splashing, pouring slime until you can neutralize the slime outlets. The story is cool, too. The slime is actually some extinct species "gift" to the universe which dissolves everything into data, transmitted to some eternal storage space at the center of the universe.

Rain world is up there with the best games of last decade.

Terraria is amazing.

Dwarf fortress is obligatory.

I guess it depends on your definition of indie some, but here are mine:

Guacamelee 1 & Guacamelee 2 - The humor is mixed but the gameplay is just so damn tight

Shovel Knight - Growing up on games like Mega Man and Duck Tales, Shovel Knight feels like it was made specifically for me.

Celeste - One of my favorite gaming experiences. Great story, great gameplay, and hard as fuck. Incredible accessibility options also.

Recettear: An Item Shop - I don't know anyone else who has played this game but it's so damn good. I love it.

Stardew Valley - The way ConcernedApe continues to add free content to this game makes this easily one of the best values in gaming, but this game would still be great even if content updates had stopped a long time ago. Have to play on PC though for mods; the default walking speed makes the game unplayable for me.

I also put years into a now-defunct multi-user-dungeon called Arythia, but that's kind of it's own whole thing so I don't think that counts.

edit: I can't believe I forgot to include Hades, which is literally one of my all-time favorite games.

• Slay the Spire: I don’t just think it’s the best deck building roguelike, I think it’s the quintessential deck building roguelike. It’s such a complete exploration of the design space of the genre in terms of the options it gives the player to build their deck and the challenges it puts those decks up against. Not that there aren’t any other fun games in this genre, but they all still feel like STS, but worse and with a gimmick that doesn’t add much.

-Will edit with more in a bit.

Terraria. My most played game on steam!

Dwarf Fortress.

Not just best indie game, but best game period.

Obligatory Undertale mention. I know it's the cliché answer, and it's fan base is...a lot, but it really is a great game.

Also, very happy to see ~~FLT~~ FTL get a couple of mentions here. Hardly any of my IRL friends have even heard of it, but it's probably the best Star Trek game ever made (even if it's not actually a Star Trek game).

Outer Wilds, and its expansion, is one of the most innovative and interesting games I've played. Made by students!

Celeste

One that I didn't see (or missed) is Project Zomboid. Absolutely can't wait for Build 42 to drop (someday).

Since I don't see it Return of the Obra Dinn is one of the best games ever made, and done by 1 guy

Since I don't see it yet...

Super Hexagon

It's one of the simplest games possible, the controls are "clockwise" and "counterclockwise", and there are no distracting characters, setting, or story.

And yet the easiest level is -- quite accurately -- labeled as difficulty "Hard". The next 5 levels (6 total) go way up in difficulty (and labelling) from there.

Each level lasts 60 seconds. If you can survive that long. I've never unlocked the final level myself, so I don't even know what it is like, but I can guess.

Outer Wilds and Hollow Knight share the spotlight for greatest games of all time. Both are as close to perfect as it gets.

Bastion gets an honorable mention. Not sure if SuperGiant Games is considered indie anymore, especially now that Hades hit big, but I love their early work.

Noita (from indie super-group Nolla games) is still amazing.

Lunacid - King's Fieldalike with a great atmosphere and PS1 era esthetic. Fun hidden secrets (sometimes a little too obscure, but whateva, still fun) that I fell in love with as a fan of the OG From Soft King's Field/Eternal Ring games.

Signalis - A thought provoking horror sci-fi game about an android trying to find their missing ship captain on a far away planet. I don't want to stay more to stay away from spoilers, but this plays homage to OG Resident Evil and other early survival horror games from a top down perspective.

Pyre - A sports game and VN hybrid made by Supergiant games. Not as popular (At least I think) as Hades, Transistor, and Bastion. Just fantastic story and world building with characters that you end up feeling so passionately for by the end of the game. Just a wonderful game and probably my favorite Supergiant game barely in front of Hades.

Dusk - A retro FPS ala Quake 1 era games. The game that kicked off the newest resurgence of "Boomer shooters" and is one of the best out there. Wonderful secrets and level design along with some solid atmosphere and scares by those New Blood boys that I love so much.

I feel like people must be tired of me recommending the same few games (you know, if anyone cared enough to read all my comments), but I'm the type of guy who is pretty much only interested in finding the more hidden gems, and I generally ignore the stuff that keeps showing up on the front page of Steam.

• The Upturned - A cartoony horror-comedy game with a great sense of humor.
• Withering Rooms - The story is interesting and the atmosphere is great.
• Your Spider - This one is possibly my favorite indie horror game.
• Exanima - Read about the features. This one is more impressive than the screenshots make it look (at least for me).
• Lunacid - I love the visual style and atmosphere of this. I also enjoyed Lost in Vivo by the same developer.
• Praey for the Gods - This one is for anyone who's looking for more games like Shadow of the Collossus.
• 8Doors: Arum's Afterlife - This is a decent metroidvania with a charming story. If you enjoyed Hollow Knight, then you may also enjoy this.

Because nobody else has mentioned it yet: Dust: An Elysian Tail.

Made almost entirely by one person, and that honestly makes it all the more impressive.

Prince of Persia (1989). Designed and implemented by 1 person. Hours and hours of fun and frustration.

Hollow knight, deep rock galactic.

Aside from FTL (which I'm glad to see is well-represented here), my top ones would probably be Papers, Please and Disco Elysium. Papers, Please manages to pair a good narrative leading to many endings with oddly fun gameplay. Disco Elysium simply has some of the best writing ever in a video game and world lore that I can't get enough of.

I also really liked The Binding of Isaac (Rebirth and later), Don't Starve, Shovel Knight, and Hollow Knight.

Terraria is the easy pick for me. I believe the only game that comes even close to the amount of hours I have in it would be Minecraft. I doubt I need to say much about this game, so I'll leave it at 3 words: near infinite replayability.

Melvor Idle is an amazing game if you like the "idle" in idle games. And if you like the idea of leveling up a multitude of different skills like in RuneScape but don't like the idea of walking back to a town every time you've chopped down 12 trees, Melvor Idle has you covered. It's a long grind but I had fun the whole way. I've 100%'d it and all the DLCs and still love playing it.

Cassette Beasts... I'm genuinely surprised I haven't seen this game mentioned here. An absolutely amazing creature collector with a very unique twist on things, a great story, beautiful pixel art, and hands down the best game soundtrack I've ever heard.

Since I don't see anyone mentions it.

Tunic. The shortcuts are so cleverly hidden that allows you to easily break the sequence in your next playthrough. The manual translation felt just like back when I tried to understand japanese game manuals that come with game boy cartridges.

Chain of Echoes. A one man RPG game with a unique combat system that has great quality of life.

Inside, Subnautica

adding Ori

Since everyone’s mentioned the standards.

I’ve really enjoyed playing Tails of Iron (metroidvania with a focus on learning bosses attacks) lately. For programming games, I really liked Shenzhen IO (You create hardware with something resembling an Assembly language and a printable manual) and Human Resources Machine (drag and drop assembly programming)

I also have a soft spot for anything from Ska Studios (I maed a game with zombies in it, salt and sanctuary, salt and sacrifice, dishwasher, Charlie murder, and a bunch of older games that are probably not playable anymore)

Also looks like everyone else forgot to mention the great game that is Undertale as well.

FTL, Rimworld.

I have about 5000 hours just on these two

Spiritfarer. Probably one of the most touching games I've ever played. What Remains of Edith Finch. Stardew Valley. Firewatch.

The "To the moon" games. They are heavly Story based but they are also one of the few games where I cried like a toodler.

Return of the Obra Dinn. City Game Studio. Until the Last Plane and Full Metal Sergeant (by the same dev).

Stardew Valley by a wide margin, probably dumped close to 1000 hours into it overall

Factorio, Don't Starve Together

Citizen Sleeper, one the the best games, not just indie.