Rather than do lists, this is probably the way to go. The (console) video game of the year is clearly one of the Halo series. It's hard to remember, given the success of the Call of Duty franchise, as well as Valve console releases such as Team Fortress II or Left 4 Dead, but there was a time when first-person shooters were almost exclusively the province of PCs. The Halo franchise proved you could make an FPS for a console system. I picked Halo 2 since it helped usher in the era of online console play as well, which is the other real turning point in the decade of gaming.
A strong argument can be made that Guitar Hero or Wii Sports ought to be the game of the decade, since they both signficantly broadened the number of people who played video games.