Which game was the original Xbox's best-selling title?

Halo 2 was the original Xbox’s best-selling title.

Developed by Bungie and published by Microsoft Game Studios, Halo 2 became the best-selling game released for the original Xbox. Its success built on Halo: Combat Evolved and was driven by a large campaign, improved technology, and Xbox Live multiplayer support.

Halo 2 was especially important to Xbox’s online identity. Its matchmaking system made it easier for players to find opponents at comparable skill levels, helping online console gaming become a mainstream feature. The game also introduced the ability to play as the Arbiter during parts of the campaign, adding a second perspective to the story.

Halo: Combat Evolved was the original Xbox’s signature launch game, but it was not the console’s best-selling title. Halo 2’s sales and online popularity made it one of the most influential games of the Xbox generation.