What's a single player PC game you can't get enough of?

Rimworld.

It's kind of funny because I bounced off the game hard the first two times I played it. What really did it for me was the Ideology DLC, gave it a shot on a whim and the amount of structure it gave me for RPing colonies was exactly what I was missing.

Hundreds upon hundreds of hours in now, and it is the game I keep coming back to. Not to mention one of the biggest modding communities I have ever seen in a video game. The only video game subscriptions I maintain currently are to a few Rimworld modders whose work I really enjoy. When I am not playing it, I am working on ideas of themed mod packs to put into it.

Dyson Sphere Program is a close second, it's my favourite factory-builder. It is still in early access but is a VERY polished experience already. Amazing dev communication too.