Criteria:
1) Nothing before 2005, because if you're doing PC gaming and still stuck on those older games, you're probably not really playing on a rig that supports the latest and greatest. And if you are, you'd probably have to run third-party emulators or other tricks to get older games to work. Not snobbery, just probability.
2) Genre matters. Gaming is tribal and not everyone likes all types of PC games. So the only games that matter are the ones in the genre you care about. That may be only a few, it may be all of them.
3) Assume all expansions are part of the title.
4) Game must be currently published in its final form.
FPS Multiplayer - Battlefield 4/Bad Co. 2, Overwatch OR Team Fortress 2
Single Player Action/adventure - Grand Theft Auto V, Red Dead Redemption, Assassins Creed 2, Arkham City, Dishonored
RPG's - Witcher III, Mass Effect (whole series), Elder Scrolls: Skyrim, Dragon Age (whole series), Fallout 3 and 4,
A/RTS - Starcraft 2, Marvel Heroes 2016, Diablo 3, Torchlight 2
MMORTS - DoTA 2, LoL
Tactical Strategy - XCOM 2, Total War: Warhammer
4X Strategy - Civilization IV, Endless Legend, Galactic Civilization III
MMO - /crickets
Casual - Slither.io (it's free!!!!!!)