Do you know any singleplayer games that are infinitely replayable?

Minecraft, Hades, Solitaire, Civ, Gunfire Reborn, Vampire Survivor, Vampire Hunters, and Placid Plastic Duck Simulator,

Hades

Doom and all the other doom clones? Prodeus, dusk, forgive me father, there's a lot

For me, it's Grim Dawn, honestly. (And Diablo before that) 10 classes to choose 2 from means a lot of new skills and builds. Tons of random loot, but also recipes to make and collect.

I mostly play with mods now. Grim Internals, full rainbow, some rep and exp gain ones, and the Item Assistant. I play solo, and have enjoyed every hour of it!

Rainworld, Cult of the Lamb, Slay the Spire, Inscryption

Classic Doom 1 and 2. There's gotta be over 100 levels if you count TNT and Plutonia, which I think were sold as Final Doom? Anyway, if you just get the base games for 1 and 2, there are thousands upon thousands of community made maps, including some total conversions, so you can play new Doom content until it physically pains you to continue.

Of course, I feel obligated to mention that even though it would be super easy to pirate the WAD files and play with a free modern source port like GZDoom, like absolutely trivial to find copies of DOOM.WAD and DOOM2.WAD floating around the net, probably showing up easily on Archive.org, but... Um... Where was I going with this? Oh, right, don't pirate. Cheap on GOG last I knew.

One I haven't seen mentioned (at a glance at least) is Noita.

Getting the "false ending" is achievable with some effort, but I dare you to actually finish the game. And as far as replayability, you'll be hard pressed to have two runs that go the same. The amount of Butterfly Effect in this game from all the combinations and systems is straight up insane.

I really can't recommend it enough.

I am currently in my like 90th run of Subnautica. I'm doing an "All Cuddlefish, no native food, save the Sunbeam" run.

What you are describing is essentially the roguelike/lite genre, lol. Search up some of that, see if anything looks fun.

Honesty I'm shocked nobody has mentioned Tales of Maj'Eyal or ToME for short. Extremely deep roguelike with story and it is getting expansions ans updates all the time.

Also it is open-source, so can be downloaded for free, but I would recommend you also buy it in steam for instance to support it.

Slay the Spire

Have to throw CK3 out as my personal favorite grand strategy game (though EU4 and the like are other options). There's nigh infinite content in weaving the story of your family and realm, and mods add a whole new layer to it.

Basically any game that doesn't in itself follow a story, so you are the story (or make it). For me personally it's building and factory games, like Factorio, cities skylines (1 or 2), satisfactory, Kerbal Space program (1 only), Rim world.

This list is essentially endless.

Doom. No the originals. Doom, Doom 2, and Doom Final. I always go back every few years.

Anything with procedurally generated levels, like roguelikes/roguelites. I can personally vouch for the longevity of Slay the Spire, for one specific example.

The binding of isaac: rebirth and dlcs

Closest I can think of to infinitely replayable games are rougelike games like Slay the Spire, Peglin, Shotgun King: The Final Checkmate, Backpack Hero, etcetera, and sandbox games like minecraft (or Minetest if you don't feel like spending money and/or don't already have a minecraft account).

Though, with the rougelike games I mentioned, there are upper limits to increasing difficulty levels.

If you're into boomer shooters, you can't go past the original doom for infinite playability. Literally 30 years worth of user created content and mods.

War for the Overworld (I have many thousands of hours in this)

Settlers 2 Gold

(Both the above have a lot of user generated maps, which has given them most of their longevity.)

Factorio

You can get hundreds of hours out of Binding of Isaac, easy

Many ppl suggest rougelike/lite and sandbox games I want to also add games with a good mod community and have a lot of side quest like Skyrim, they fit the replayability criteria because when it starts to feel the same can you add mods that change things up. I have done thousands of hours in Skyrim and never finished the main quest 😂 I think Balders gate 3 will also live for a long time. Many rpgs seem to get a lot of mods and games like Balders gate change a lot depending on what you do and how you play.

But you seem to want some kind of fps so warframe would be better, you can play alone or with friends. Just like ultrakill do you jump around killing, you can use, swords, guns and magic depending on your build (there are many). Doom would also fits your style of playing I think.

Otherwise management games is a time sinker. But most do not have fighting elements where you yourselves fight. Cult of the lamb tries to be all of it, it has action/fighting and management you can even decorate if you are into that. Pretty good for those who want it all. The devs still updates it too.

I've been recently playing the "so called" sequel to ftl, ftl multiverse. I have gotten the standard and old secret endings, but there are hints of a third even harder to find ending.

The binding of Isaac.

I've been playing it on and of since the flash game released 13 years ago.

Deus Ex

Rimworld would be my top suggestion, as others have noted.

I picked up Old World (excellent native Linux support BTW) during this summer sale and have not been able to put it down. If you're a fan of Civilization style strategy games I'd highly recommend checking it out. I haven't really enjoyed a Civ game since Civ 4, and Old World feels very similar but fresher and with less jank. it's got a Crusader Kings style dynasty system with randomized events that adds a layer of role playing your leader and securing their dynasty through heirs you can train/influence.

As for the repeatability, Old World has tons. Each culture plays significantly differently, and each leader has different bonuses that encourage an interesting style of play. Games don't play the same because of the mentioned event system, but also because learning new technologies is "randomized" as well. New techs are researched based off a selection of 4 drawn tech cards once you finish a previous technology. The card system makes it so you can't just rush straight to archers and dominate the early game to snowball into a power house every game, but its not truly random so you can "game" the system in your favor to get the techs you want with the tools the game gives you through either unique leader powers, or specific governor roles for example.

The game is super deep while not being off puttingly complex.

Europa Universalis 4. And EU5 is on the horizon.

Definitely not like the games you've mentioned though.

Generally games with random elements are considered to be good for dumping tons of hours into. So games with randomly generated worlds like Minecraft, roguelikes, strategy games that are always variant just because of the nature of AI actions always being a little randomized, and other stuff like that. So maybe like Minecraft, Dwarf Fortress, Crusader Kings 2 or 3 as like a basic list. But really the game that's going to be the most replayable is the one you don't get tired of. I've beaten Thief: The Dark Project hundreds of times and that game is a relatively simple level-based stealth game with no random elements and not even especially huge levels.

If you don't mind top down colony managers, rimworld is an absolute gem. The base game itself is infinitely replayable because every game is basically a story. Think dwarf fortress with graphics (well, df has graphics now but). Also incredibly easy to mod to customize or completely change your game experience.

Strictly speaking about single player games, as in no rogue likes and such, Max Payne 2 is probably the game I played most times over the years.

Honestly, Skyrim. With so many great mods available these days, you can literally turn it into a whole different game. And with modpacks on Nexus, Wabbajack or similar sites, it's easier than ever. I'm just getting back into it myself after 3 years, but I'm still a big fan of getting into the nitty gritty of it and configuring everything myself.

Look up project brutality or brutal doom. Those are great doom mods with a lot of fun gameplay. I used to play that for hours.

The Anno series is pretty cool. It's like playing crack. I've been playing 1800 recently and it feels like an instant classic.

The elder scrolls games are great for this. The further back you go the more replayability there is IMO. Morrowind is goated.

Dungeon keeper 1&2 are both a lot of fun, and have lots of custom maps. The original dungeon keeper even has a full engine rewrite which is really good.

If you want to try map painters, Crusader Kings 3, EU4, and Victoria 3 are all excellent and in depth games, with a lot of replayability stemming from all of the different ways to build up your nation in the sandbox.

Rimworld, dwarf fortress, etc. Have fun!

Diablo 1/2

Dyson Sphere Program is dangerously replayable to me. Hundreds and hundreds of hours sunk into it

Dynasty Warriors 2, Dynasty Warriors 3, Dynasty Warriors 4, Dynasty Warriors 5...

Super Metroid. The game really opens up if you spend a bit of time learning alternate routes and sequence breaks. Many of them are kind of easy too. Besides all that, I just feel weirdly cozy in the depths or outside in the rain. It's also a shortish randomizer if you go outside of the vanilla experience with like 100 places to check.