You can't beat Half-Life 2. You just can't. Even by todays standards it is still one heck of a game. It also had a serious impact on the industry as a whole. Although there were quite some heavy hitters released in that year, none of them comes even close to the brilliance or significance of Half-Life 2 if you ask me.
Of course the choice is always subjective. But for me, this is an easy one.
Official member of VGC's Nintendo family, approved by the one and only RolStoppable. I feel honored.