Wow, that's a difficult decision. Umm...
1.) Team Fortress 2
2.) Killing Floor
3.) Dreamfall Chapters (release this year, please!)
I can't really decide if these would be sufficient, though. Well, those are my choices. Custom maps made by the community and future DLC ensure that Killing Floor won't ever get old, I still haven't gotten bored of the Team Fortress 2 map 2fort(and the DLC), and anything related to The Longest Journey is worth quite a bit of interest on my part.
I really wanna play a single-player game again, though. After I started up PC gaming a little over a year ago, I haven't been able to avoid multiplayer games, and they've prevented me from playing my single-player games.
I personally love Tim Schafer/Sam & Max adventure games, though, and they are all on PC. Hm.