Diablo 2, and Age of Empires 2 after it.
Since CS is not a PC exclusive, I'm going with System Shock 2!
---Member of the official Squeezol Fanclub---
Modern PC games: Civilization 5 followed by Anno 1404 (although both were released already years ago).
Back in the day: almost all LucasArts adventure games, Civilization 1 and 2, Dungeon Keeper, Dune 2, Outcast, Age of Empires 2, Warcraft 3, The Settlers 2, Baldur's Gate 1 and 2 and many more.
Overall: probably Civilization 2.
Company of heroes:Tales of Valor...i think.
I don't think I have many specific favorites anymore, on any platform. Some of my favorites include the Civilization series, the Total War series, Titan Quest, Wolfenstein - Enemy Territory...
Fallout 3 and Fallout: New Vegas.
I'd probably go with Starbound, Contagion, SCII, Diablo3, HOTS and AOE series.