Definitely Halo. Also the Xbox was a very good console. I still have my OG Xbox. Then again...I love my GameCube. Funny I loved those 2 consoles more than my PS2, although I had most of my games on the PS2. Sorry I started rambling. :-l

They stole the first person shooter crowd that N64 had with Goldeneye/Perfect Dark.

The GC VS XBOX was a pretty nice battle... funny too, At first GC was above XBOX, the XBOX surpass GC, then GC surpass again XBOX, and then XBOX surpass again GC...
This was after PS3 VS 360 the bigger close console war ever. 

And was a very nice battle in both Globally and in the US. in 2003 GC has outsold XBOX not only globally, but even in the US:

It was more than Halo. Live was brand new. They had Peyton Manning pimping the new NFL Sports game. Shenmue was released(1&2 again). The system was brand new. Platforming and party games were apart of it as well. Listen to music you stored while you play your games. Halo was main no doubt but there were other very good factors.

It was first console that was very Western-centric - for people who were not already into PC gaming, that was something very different than usual console offerings.

Well, according to VGC Halo 2 sold around 8.49 millions, which means at least 16 million Xbox users didnt buy the console because of it. So there has to be something  more to it.

It is also worth noting that the XBox only had two games above the 4 millions mark, whereas the GC had four.  Also, Super Smash Bros. Melee is not that far behind Halo 2 (and actually it sold better than Halo 1).

So like I said, theres gotta be something more to it.

because it wanted to

People say Halo but there is more then that. A little port named Grand Theft Auto maybe. VVestern oriented games. PC oriented games. Xbox live.

I agree.Yeah halo was big, but there were many factors that sold the console.I never owned an Xbox up until the XB1, but I remember being really impressed with all the homebrew the console had.To me consoles were just about putting a disc in a console and playing, so it really blew me away when I saw all the things the console was capable of , since at the time I just had no idea how it was doing them.I really do believe that homebrew may have been one of the reasons of why it outsold the GC.

The Master Chief