Deftinately Borderlands. I would never say a game is perfect, and I tend to be a bit, over critical. But for the first time I can say Borderlands is the first game, which I think comes really close to the perfect shooter.
I'd say games of BioWare are the best. They have great story, elements of mystery, detective, horror, logic and so on.
I still play halo and halo 2 for pc, and there are still an abundance of users to play online against
So many good PC games it's hard to choose... ME2, MMOs like STO/WoW/Aion/RoM/GW etc, and a probably a ton that I haven't played yet, but plan on playing. My all-time favorite game for PC has to be Unreal Tournament, though
If I'm going old school and just want to pick a video game.....Galaga. If I'm specifically picking a PC game...KOTOR