If I could only have 5 PC games until the end of time.

These are the PC games that, if I were force to limit my total game count, I would say are the ultimate, must-haves. 
Each of these games scratches an itch that no console could replicate. No matter how long I'm going to sit down, I could find something awesome to do if I had these 5 games. See comments for each for a quick summary of why I love it. Know that none of these would work on a console quite the same way as they work on PC (save maybe Audiosurf, as I play that with a X360 controller, but my whole music library is on my PC, not my Xbox).

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