When it comes to oldschool, there's lots of re-playable classic gems, some forgotten with the death of DOS and inability to properly run on a virtual instance.
But for what is more accessible and not console, CS:GO is still and will forever be a king in FPS. Battlefield 1942 was the ultimate version of BF. Quake is honorable mention, but with champions now it's got new life.
I did not know there was a really active Neverwinter Nights modding community, I might have to check that out. RPG wise, it's really a question of taste, lots of classics to spend hours on if one is so lucky to have the time.
Going from newer to olderish (probably not perfect chronologically:
Minecraft
Half Life 2
Roller Coaster Tycoon 2
The Sims
Diablo I + II
Dungeon Keeper (gosh I wish they had made a real sequel instead of a mobile junk game)
Simcity 2000
Doom
The Oregon Trail