This year I have played all the main contenders :D
I actually have to give it to COD 4. Bioshock comes close with its world design, atmosphere, story and interesting gameplay, but the simplicity and minimalist design of COD 4 just does it for me. A short but pretty much perfect campaign in my opinion. Also love Portal, which was a game changer. Halo 3 is a little overrated, I prefer CE, 2 and Reach. God of War 2 probably deserves more recognition.
Super Mario Galaxy is of course great, but people tend to forget that you have to put 4-5 hours into these modern Mario games before the levels get interesting and just a bit challenging.