Original: After its initial release, Halo 2 was the most popular video game on Xbox Live, holding that rank until the release of Gears of War for the Xbox 360 nearly two years later.
Simplified: It held that title until the release of Gears of War nearly two years later. From the day of it's release until 2006, 2 years later, Halo 2 was the most played game on Xbox Live.