Skyrim. I just can't get away from it and keep finding mods that I didn't know about. It's a nightmare. And Command and Conquer. At some point I want to get through everything on the highest difficulty level, but my goodness is it crazy unfair.
Neverwinter Nights.
Whenever I want another fix of DnD 3.X (my favourite of all the dnds), I do another run. I got all the expansions too, so there's "variety" there.
outside of some NSFW, the only game I have installed on this computer is Sims 4, but this isn't my gaming computer. Those one has something games like Halo, Mass Effect, Slime Rancher 1 and 2, The pre origins Assassins creed games, and maybe some other's I can't think of at the moment.
Not always installed, but sort of a Skyrim installation (and mainly, millions of mods) are always sitting there, waiting for me to return.
And CK2 or CK3.