I really don't have a single favorite game for the PC. I love all of my PC games equally.

I guess if I had to choose, it would come down to Starcraft and Medieval total war 2.
Starcraft was a revolutionary game for it's time and I hold it dear as one of the best RTS games ever created. With such a fine balance of each race, it sets the standard for other RTSs. The big things that keeps me playing is battle.net and the map maker(s).
The total war series all together is golden and doesn't need explaining for just how awesome and well done it is put together. I used to love Rome total war. And frankly, I was reluctant to let go of it and move on. But Medieval total war is even better. It kept the things I loved about Rome, but fixed all the bugs it had.