1. Batman: Arkham Asylum
Surprise hit of 2009 for me. I heard very little about it and read very little about it. I got it free with my new video card and can't believe how close I came to missing out on this fantastic game.
2. Uncharted 2: Among Thieves
I never played the first Uncharted, but I plan on it after playing this. Really well made, it was a close race between this game and Batman.
3. Left 4 Dead 2
I was a big fan of the origional Left 4 Dead. I knew I'd be buying part two when I first heard about it. It was a surprise that it came so soon after part 1, but with additions to the game it was well worth it. I ended up putting a lot more hours into L4D2 than I did in L4D. Valve knows how to make awesome games.
I have spent a lot of time rolling through Borderlands. I think it was overlooked by some and doesn't get the credit it deserves. I was surprised at how well the RPG and FPS aspects worked togeather in this game. Even after multiple playthroughs this game is still a blast, when playing co-op at least.
5. Dragon Age: Origins
One of the better RPGs I have ever played. Visually a very good looking game, and the gameplay was addictive. I think for me it was almost too long, by the end I just wanted to get done with it, though the story played out really well.