Interesting question. I'd say the three most revolutionary games of the 21st century are Halo: Combat Evolved, Grand Theft Auto III, and Wii Sports.
Since GTA is multi-platform, it's down to Halo and Wii Sports. Although Wii Sports turned the industry on its head in 2006, and stands today as the most "killer" of "killer apps" in the history of video games, it's influence has petered out over the last few years. The Wii U failed to capitilize on the revolution that was Wii, and Wii Sports, once a marquee title, has lost its luster. Motion gaming in general, unfortunately, has been more or less abandoned outside of Nintendo.
Then there's Halo, which, along with its sequels, revolutionized console shooters and console online multuiplayer, and influenced two generations of first-person shooters. Its control scheme, weapon inventory system, and focus on militarism has become the standard for shooters. Furthermore, the series is still going strong and, more importantly, its legacy is still going strong.
So I give the edge to Halo.










