FreeCiv, single player mode. A
lot of
FreeCiv. And occasionally
FreeCol. I'm terrible at them, but I play them on easy mode, often leaving them in the background for days at a time and dropping into it from time to time for a turn or two, as a means of distracting myself between other things.
Aside from that, I have a number of games, including several in my Steam account which I haven't gotten around to playing (e.g.,
The Stanley Parable,
Psychonauts,
Never Split the Party). Many of them are somewhat older, including most of the ones I've actually played. I have yet to finish most of the ones I have played, assuming they have an endgame at all.
Top among the ones I've actually played are
Fallout (as in, the original 1997 game),
Fallout: New Vegas,
Half-Life 2,
Portal,
Thief: The Dark Project Gold edition (again, the late 1990s game, not the reboot),
Hearts of Iron IV (which I only just started and am truly awful at so far), and
Crusader Kings II (which I can't for the life of me figure out). I used to play TF2 a fair amount, but whew, that community, talk about toxic...
I played a little of
Undertale,
Pony Island (which is not at all what the title would lead you to expect), and
Bendy and the Ink Machine, but got stuck and/or distracted halfway through each. They are good games but I never got back to them for one reason or another.