Oblivion

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.

Metal Gear Rising Revengeance

Cataclysm: Dark Days Ahead

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.

Rimworld and factorio cause time to disappear

Minecraft I keep coming back to year after year.

FTL is pretty fun too

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

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

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

Factorio and rimworld

Recently, The Stanley Parable

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

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

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)

Europa Universalis IV

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)

dcss

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

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.

Two Point Hospital

Stardew Valley!

Space Engineers with my 1.5k hrs of played.