Not a fantastic game, but the first Walking Dead Telltale series really hit me in the feels and so has stayed with me. The Witcher 3 definitely affected me, but it's hard to seperate that feeling from the books.

I'd say Kerbal Space Program, Factorio and Rimworld because whenever I play those it's like coming home, but since I'm always playing at least one of them, it feels a bit like cheating.

The feeling of the Tribes games is always there in my mind. I occasionally go back but the freedom of movement in that game is just so liberating. I never know why they aren't more celebrated. (Titanfall 2 comes close, but still not quite the same).

Battlezone 2 is IMHO another unsung classic. I loved the mix of RTS and FPS. Another one from before the mechanics of FPS and the general feel of motion had standardised (to a fault) which I think helps it keep it's hooks in me.