Crusader Kings, Europe universalis and imperator Rome. I know this is three games but it's the same general idea, as a history fan, this game scratches a very specific itch for me

There's also bannerlord/warband, amazing experience, sorta like ck3 on a micro level

Stardew Valley!

Ring of Pain. It's a creepy dungeon crawler roguelite. I don't know why, I've seen everything the game has to offer and somehow I keep coming back to it.

Another vote for Vampire Survivors. It feels like a game that has given the developer so much joy to make.

I love to start a new round of Civ 6 or a create a new city in City Skylines, because i learned so much from my previous mistakes and this time i will create a perfect civ/city... only to make super stupid mistakes nonetheless and fail miserably. It's still great fun though.

Metal Gear Rising Revengeance

I'll still install Duke Nukem 3D every couple of years to replay or to play the custom maps that were so easy to make.

OpenTTD is also great to mindlessly play while listening to podcasts.

Oblivion

Monster Hunter (I rarely play multiplayer), MGSV and Xcom 2

I know it's not 100% single player, but it's how I play it anyway, D2 and all of its mods.

Technically not a single player game but Elden Ring.

Europa Universalis IV

Vampire Survivors, The Binding of Isaac and Civilization 6, absolutely

Grand Theft Auto IV. I've always felt it has the most emersive storyline and setting out of them all. Once you get past the janky driving controls it was a hell of a lot of fun.

Dwarf Fortress and Minecraft. I don't play the latter online at all.

It's not really a single single player game, more a set of specific genres: "Ubisoft open world", "Immersive sim" (especially Arkanes), "Bethesda RPG" (Even 76 which ye cna play pretty much solo), "Walking Sim" (a genre I fell in love with this past year)

Enter the gungeon. Played it so many hours across different platforms.

Most of them that I play since I'm not big on multiplayer. Lots of RPGs (Mass Effect Legendary), colony sims (Rimworld), city builders (Banished), and grand strategy games (Stellaris)

Recently, The Stanley Parable

In the category of "bullet-heaven" / vampire survivors I have been putting a lot of time into Bio-Prototype. It has a really amazing upgrade/combination system for your weapons. And it also only costs like $4