I'll give you a list of what I've been playing and think is good:
Single-Player:
Assassin's Creed 2: Think Prince of Persia + Splinter Cell + Legend of Zelda: Ocarina of Time. It's an amazing game, and they just removed 90% of the bad DRM.
Just Cause 2: Get this because it has some of the best graphics on PC right now. It looks amazing on my new gaming PC. You'll really appreciate your investment in a gaming PC when you see the beaches, or snow covered mountains as you glide across them with your parachute. And while it's a pretty average game overall, it has its fun moments.
Audiosurf: If you like music at all, get this. It's a very fun way to listen to it.
Fallout: New Vegas: Want a game that will take up hundreds of hours of your life? This will do it. I doubt I'll ever complete over 50% of it, it's so gigantic.
Europa Universalis III: A solid RTS, where you command an entire country versus other countries from the 1400s to the 1800s. Multi-player is practically nonexistent unless you organize it on the Paradox forums.
Mafia II: Ever wanted to live in the 1940s and 1950s as a mobster? This game creates a great world for you to do just that. Be warned that it's a story-based game. Yes, there is an open world. But it's a linear game set in a non-linear world, something that most idiot "professional" game reviewers didn't see. They just saw an open world and decided, "HERP DERP IT MUST BE SANDBOX OR ELSE!"
Multi-player:
Starcraft II: It's a fun multi-player game with friends. The custom games are always good. The single-player was ok, too, with fun missions but the cheesiest cut scenes and story I have ever seen in a game.
Battlefield: Bad Company 2: A fun FPS, with larger maps than CoD. Of course, you can (and I do) use shotguns and close range weapons. Plus, the buildings blow up. Dodge the single-player on this one unless you have lots of spare time.
Call of Duty: Black Ops: Forget the haters. If you like FPS games at all, you should love this game's multi-player. Tons of customization, an RPG style system of leveling, and constant close-range combat. Dodge the single-player in this as well, because it's very average. And I found the first level to be plain offensive.
Counterstrike: Source: Hey, it's fun if you only have couple quick minutes to kill. Hop on a mini-game server, or try Jailbreak.