Crysis 1. Almost everyone only cares about it for its graphics and says the gameplay is pretty mediocre or boring, but I've played the campaign through 5 or so times and still enjoy going back and messing around in different scenarios. Crysis Warhead is good too but it's more on the linear side.
STALKER if you want a longer SP experience with a lot more depth. Think of Fallout... but a lot more intense.
The Orange Box: Half life 2, Episode 1, Episode 2, Portal (get you ready for Portal 2!!!), Team Fortress 2... For $20 on Steam right now. How can you go wrong?
TES IV Oblivion: Get you ready for Skyrim! Plus this one has a load of mods to extend gameplay on the PC.
Metro 2033: It's like Stalker but a lot more focused and linear. If you've got a machine up to the task it has interior graphics unrivalled on any platform.
Cryostasis: A lot like Metro 2033 in terms of being very atmospheric (and a resource hog). The game is pretty buggy though and the hardware requirements are through the roof in comparison to what is actually going on in the game... so I'd look it up some more before committing.
Starcraft 2: Need I say more?
I'm running out of time so I'll just list the rest without description and priase
And that's just off the top of my head. There are a few notable free to play titles out there and sequels to a lot of the games I listed above coming... Not to mention MMO's like Rift, SW:TOR, Guild Wars 2...
Also if you get a chance and want to share some of your PC gaming bliss please do contribute to the PC Gaming Screenshot thread. ( http://gamrconnect.vgchartz.com/thread.php?id=127105 )