Gaming

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.

FTL. It’s a simple fun cheap game. Steam says I’ve played this for over 3,000 hours!

https://subsetgames.com/ftl.html#_

Besides what people have mentioned, you also have simulator type games like SimCity. Though with SimCity, I got bored of the "new" SimCity they released.... in 2013. Either play something like SimCity 4000, or try Cities Skylines.

Hades

UT99 so shoot me lol

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

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.

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.

Football Manager 2016 was the first version I tried. I put 1,500+ hours into it and only stopped because I decided to upgrade to Football Manager 2018. I'm on 2024 now at ~400 hours.

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.

Turbo Overkill is sort of Duke Nukem/Quake strafe boomer shooter with modern schmovement mechanics like Ultrakill.

Also another vote for Prodeus. It's basically a Brutal Doom clone. It's messy and gory and just a tone of mindless fun.

I'm playing Prey right now and it's also a ton of fun trying to figure out ways to do stuff instead of running in all guns a-blazing. Still trying to figure out how stealth really works mechanically.

So maybe don't take this as a real suggestion because it doesn't make sense to be a game with lots of replayability, but I've replayed the everything shit out of FF8. I don't think it's because of the game. It's because of me. Maybe it might do the same for you?

My first reaction on seeing the title was to name a games that just have fun core mechanics where replaying the game is basically just doing more of the same fun thing, so ULTRAKILL means I'm on the right track.
Prodeus is a pretty good doom clone. Not as in-depth as ULTRAKILL, but nonetheless has fun weapons and your basic doom (eternal) movement/mechanics. Blasting your way through enemies is as fun on the first level as the last. And the Quake games hold up extremely well, and the mechanics are so simple and powerful that you can really have fun replaying and getting better.
Stealth games are also very fun for this. The Splinter Cell series (especially Chaos Theory) are very fun to try to perfect/improve on. Dishonored and Thief as others have mentioned. And Midnight Club is the best racing series for this. You'll have to emulate it, but it's worth it. Completely open-world, and learning the city layout over time is very satisfying.

Obligatory Rimworld shout out.

I've dumped more hours into this game than the rest of my game library combined.

Edit: typo

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.

Spelunkyyyyyyy!!!

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.

I’ve played Risk of Rain 2 on and off for years and I’m not bored of it yet

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.

Doom? Does it have any wads you can play on... like myhouse.wad...

Deus Ex

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/

Civilization.

Fallout: New Vegas. Skyrim. Especially with alternative start mods.

I feel like the best options would be strategy such as CK3 as the other commenter mentioned or endless sandbox games like Minecraft and Euro/American Truck Simulator. X4 Foundations is a pretty fun space sim, and there are the Bethesda games with mods, Skyrim and Fallout 4 have some pretty cool mods and eventually Starfield too.

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

Wart Under

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.

Diablo 1/2

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!

Xcom, xcom2

I almost exclusively play single player games and honestly Elden Ring has been a huge time sink. There’s just something about mastering it that is satisfying. It has online features but they’re not required.

Some single player games I've replayed often that aren't roguelikes would be...

Dishonored Star Wars Knights of the Old Republic 1 and 2 Prey 2017 Hitman, but specifically the World of Assassination games Bethesda RPGs Grand Theft Auto/Rockstar, specifically for me 5 or Red Dead Redemption Dark Souls (I replay it on offline mode predominantly anyway) Dying Light Middle Earth Shadow of Mordor/War Halo MCC campaigns Mount & Blade series Katana ZERO Vanquish/other platinum games

Minecraft?

Hard to do better than the OG endless sandbox.

Tons. There's an entire roguelike genre built around this; some of my favorites are Vagante and Streets of Rogue. There are games with procedurally generated worlds like Terraria, RimWorld, Dwarf Fortress, and Factorio. There are RPGs like Baldur's Gate 3 that have so many ways to spec your characters and so many permutations of how events could unfold based on what you did that you're unlikely to see them all.