CoD: Modern Warfare 2

Infinity Ward's Call of Duty: Modern Warfare 2 has broken all previous records for day-one game sales. According to the sales figures released by Activision, the game sold 4.7 million units in North America and the United Kingdom alone in its first 24 hours. This exceeds Grand Theft Auto IV's record by 1.1 million units.

Mike Griffith - "The Call of Duty franchise has become a cultural phenomenon showing the power of video games as an entertainment medium. The shattering of these entertainment records is a testament to the compelling, cinematic and uniquely engaging experience that the Call of Duty brand delivers. Modern Warfare 2 has taken interactive experience to unprecedented heights setting a new standard for entertainment."

COD4 (Call of Duty 4) sold 7 million units in 2 months, so MW2 is doing really well. At this point it's unknown how many units each version of the game sold, but it's pretty clear that the Xbox 360 version is on top.