In addition to RimWorld, I would say Deep Rock Galactic has never even once left me in a worse mood after playing it. Every time I boot it up, I cannot stop admiring the love and care GSG has put into their game and almost all of it for free. The only monetisation in this game is purely cosmetic, but they make sure that the stuff you can get through gameplay alone doesn't look worse than the stuff you pay real money for. Add on top of that the awesome gameplay, atmosphere and community and you got a game that pretty much never fails to lift you up.

If shooting native lifeforms as an exploited space miner isn't quite your mood right now, you could also try Tchia. Super pretty and lovely game, although I suspect it doesn't offer too much replayability, once you completed it. But the same could be said for Okami and that is the game I first say when asked what my favorite game is.