Gaming

Dungeon keeper.

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

Diablo 1/2

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.

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

I put multiple thousand hours into Mount blade warband, Crusader Kings 2, Europa universalis 4, and more.

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.

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.

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

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.

Red Dead Redemption 2, Fallout 4, the BioShock series

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

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.

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)

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

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.

I have enjoyed caves of qud. Feels like every run is brand new.

XCOM! ! !

Slay the Spire, with mods

I was about to suggest "Balatro" but you don't want infinite dopamine game then play The Binding of Isaac:Rebirth.