How about a randomizer? Pick a game you like and see if there's a randomizer for it:

https://video-game-randomizers.github.io/rando-list/

Cataclysm Dark Days Ahead

https://cataclysmdda.org/

Zombie survival roguelike. It's free, open source, and a mature project.

Kerbal Space Program, for sure. Skyrim is another.

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.

For me, Noita. I don't recommend it unconditionally, but for me that game will forever be the only permanent game in my library. I expect it's possible that I could finish Elden Ring. I know I will never finish Noita.

Fallout: New Vegas, Caves of Qud, Project Zomboid, Minecraft, Terraria, Morrowind, Skyrim, Dwarf Fortress, Kenshi, Rimworld, Elden Ring, and so much more.

Civilization.

Planescape: Torment is extremely replayable. I've been playing it every few years since I got a copy in I think like the early 2000s. It may be that this has something to do with having gotten to play it a little bit in the 90s but not having gotten to play the whole thing. There was a lot of anticipation there.

But I don't think it's just that. It's incredibly responsive to choice, and it's one of the first games I can recall with things like faction reputations and alignments. There's a lot there to dig through, and even once you have, it's always cool to wander around Sigil. It feels very alive.

The other one I end up replaying over and over is Shadowrun for SNES. That's not so much infinitely repayable though as just a really great game that I'm happy to run through.

This list is GREAT ... but if you'd be willing to venture back to the late 90s or early 00s I'll offer you three more names:

1. Fallout 2
2. Fallout
3. Mafia

All three are open world.

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

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.

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.

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.

Europa Universalis 4. And EU5 is on the horizon.

Definitely not like the games you've mentioned though.

For the most enduring single player experiences, you really should give some grand strategy games a try, like older Total War entries, Crusader Kings, Civilization, Swords of the Stars, etc. Stuff like Factorio or Cities Skylines can also become addictive, but none of these games is action oriented.

Also, Age of Empires 2. I haven't played 4 yet, but I think it speaks volumes that, at least on steam, AoE2 has more players at any given time than 3+4 combined.

Ziggurat might be an interesting pick, as it's a roguelike FPS with magic weapons, though it might feel super neutered compared to Ultrakill. Risk of Rain 2 is 3rd person and roguelike, so every run you start from scratch and enemies will keep spawning at certain intervals, but it's a fine shooter

Fallout 4 might have a rather clunky shooting, but if you get into it, you can spend many, many hours blasting a variety of enemies, finding all sorts of places and weapons. Skyrim is a close second, while better played in 1st person, it's medieval sword and sorcery.

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

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

Shattered pixel dungeon

For me Rollercoaster Tycoon 1&2. But you could probably throw most Sim Games into here like SimCity 2k 3k or 4.

Borderlands 2 has a lot of replay potential without getting boring. It never plays the same way twice. The weapon drops are very different each time through. Don’t forget the DLC. The rest of the games in the series are fun too, but BL2 seems to be where it peaked for me.

Stellaris, Rimworld, the Sims, and a lot of stragedy games.

Stardew Valley

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.

I don't know how much is truly infinitely replayable outside of tetris, but for what it's worth I have 250 hours into dead cells and I still haven't beaten 5 cells or even reached the final biome. You'll definitely get replay value out of it, the game is fantastic.

My most played games outside of actually multiplayer games are arpgs. Diablo(mostly 2 and 3), path of exile, last epoch. Diablo2 and last epoch can be played offline, PoE can't. And yes, they're technically online multiplayer games but most people play them alone so they might as well be single player games to me. After that my most played games are RPGs and rogue likes. Plenty of good suggestions for that already

Dungeon keeper.

Any of impression games latter worldbuilders (pharaoh, zeus, emperor)