Mass Effect 2 - It took an already great formula and improved on it. I loved what they did with the gameplay, ammo, and the removal of the Mako. It had a great story, interesting characters and was just an amazing game.
I did miss Wrex though...
God of War 3 - God of War is the type of series where more of the same is a great thing. The word 'epic' is used far too loosely in gaming communities, but I think it aptly suits GOW3. Everything is just so large scale, the score so fitting that it makes me feel like I've accomplished something moreso than other great games. And Hell, it's incredibly fun.
Mario Galaxy 2 - Though I haven't started it yet, I'm assuming it's similar to its predecessor in fun factor; if it is, then it's an obvious candidate.
Honourable mention to FFXIII - Though I thought it's battle system to be the best in the series, it suffered from mostly disinteresting characters.
It really has been an astounding first half-year.
