NPD, GfK Chart-Track and Enterbrain, companies which track consumer sales data in the US, UK, and Japan (respectively) have combined their data to determine what the best-selling video games in the world were in 2008. Here are the top five:

• Mario Kart Wii (Wii / Nintendo) - 8.94M (5M US, 1.94M UK, 2M Japan)
• Wii Fit (Wii / Nintendo) - 8.31M (4.55M US, 1.61M UK, 2.15M Japan)
• Grand Theft Auto IV (PS3, 360 / Rockstar North) - 7.29M (5.18M US, 1.85M UK, 256K Japan)
• Super Smash Bros. Brawl (Wii / Nintendo) - 6.31M (4.17M US, 404K UK, 1.75M Japan)
• Call of Duty: World at War (PS2, PS3, 360, Wii, DS / Treyarch, Rebellion, n-Space) - 5.89M (4.46M US, 1.43M UK, 0 Japan)

Granted, these numbers don't keep track of every gaming market in the world but the US, UK, and Japan are the three largest so it's quite likely this ranking would hold true even if every country's sales were factored in. The fact that two games made it onto the list with little or no sales in Japan (Grand Theft Auto IV and World at War) really demonstrates how huge the Western gaming markets have gotten. It also explains why Japanese companies like Square Enix have lately taken a greater interest in producing games geared toward the US and Europe.