Simple: post what you consider your favorite PC game ever and list your reasons. If you have a hard time thinking something up, a good way to judge is how often did you play the game? Were you good at it? Did it challenge you? Did it have a satisfactory ending in its story line? Did you like the online community? Did it have good replay value?
For me personally, I'd probably have to go with Empire Earth. The game is dated now, but out of all the games I've ever played it probably kept me occupied the longest amount of time, I played almost nothing but EE and its expansion for an entire year. In fact, I DIDN'T play anything but EE and its expansion for an entire year. In this game I feel I can honestly say I had a large amount of skill, in the last single player campaign the online community was hard-pressed to discover an easy way to beat it (especially the last level) and after playing multiplayer for a bit I beat the entire thing with relative ease. I won at the very least a 2:1 ratio online against a fairly small community, and I played it constantly. I have not yet had any such experience with any other game and I doubt I ever will again.