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The one I never get tired of replaying is Mafia 2. Maybe a weird choice since it's very linear and every play through is pretty much the same. But the story is just perfect for me. The gameplay perfectly suits the story, nothing especially clever, just well executed.
Then there's games like Binding of Isaac or Dead Cells where every play through is different.
X4 foundations and relevant DLC. I have multiple saves with over a year worth of playtime
Shotgun King.
<3
Yeah Shotgun king is nice and is also very moddable too
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
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.
Factorio and RimWorld immediately come to mind. Even with the base unmodified game, you’ll likely get several weeks of gameplay out of Factorio. Then if you dive into modding, you’ll never put it down. Multiplayer is really nice too. And their big DLC just got announced, and is planned to drop in a few months. So now would actually be a great time to dive in, because you’ll get access to the DLC about the same time that you’ve burned through the content on the base game.
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 have enjoyed caves of qud. Feels like every run is brand new.
I was about to suggest "Balatro" but you don't want infinite dopamine game then play The Binding of Isaac:Rebirth.
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.
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.
I’ve played Risk of Rain 2 on and off for years and I’m not bored of it yet
Just Cause 3, with the additional DLC. I've been through all of it three full times, slowly plugging through a fourth (when time allows)
Red Dead Redemption 2, Fallout 4, the BioShock series
I want to throw out Mario Bro 3, Tetris, Dr Mario, and Sonic Mania. Just pick up and go, no long term commitment, always fun solo but you can add a friend anytime. Simple but rewarding game play loop.
Rally racing games; special stage rally is racing on a closed road and competitors start one at a time, competing for the fastest time at the end. These games are infinitely replayable in that you can always try for a faster time. The arguably best rally game is Richard Burns Rally, which has been kept alive by modding communities like the Rallysimfans.hu plugin, where independent developers continue to create new tracks and cars. Other fun rally games include Dirt Rally, Dirt Rally 2.0, Dakar 18, and BeamNG.drive (which has a massive modding community of its own) Special mention to the Gran Turismo series and Wreckfest, which both include rally among many other types of racing.
Heroes of Might and Magic - the originals. Mostly 3, but any from 1 to 5 can do it.
6 and upwards, it's just pure Ubisoft enshitification. Can't even play them properly anymore due to the online component having shutdown its service.