Definitely Modern Warfare 2. You can say it was more of an '09 game, I don't care. Not only is it the best game in the CoD series, it's also one of my most favorite FPS of all time. Other good games I played in 2010 were Assassin's Creed II and Brotherhood, also Fallout 3 (but that ones REALLY old, I just happened to buy it in 2010 though).
The favorite game of mine in the year of 2010 is by far halo 3. That game was my life and will always be my life. I absolutely was a nerd for it although now, halo has died out which makes me a little mad.
lots of standard replies of "call of duty" which can be expected,
but for the real game of the year last year, get Battlefield 2
and the runner up would be Red Dead Redemption.
AC2 was actually amazing too, a huge step up from AC1,