What game defines the xbox?

I'm not sure what you mean by "defines the 360," but the Xbox brand was built on the back of Halo. Without Halo, we wouldn't have Gears of War on the 360. It would probably be a PC exclusive game. Jumping into Halo 3 as your first Halo game, especially MP, will not generally give you a good experience. It would be like never playing baseball and then trying to step up and hit a ball off of Roy Holliday. It probably will not work out for you. Halo is not a twitch shooter like the COD or Battlefield series. It takes either many more bullets, or precise aiming to kill someone. And as such, lends itself to the player that puts in the time. I do love Gears of War, but the online in the second one was and still is an unplayable mess unless you are the host. I do have high hopes for GoW3 though since I have heard word that there is supposed to be dedicated servers.

Xbox = Halo. For me personally when I think of my 360, I think of Call of duty.

Agreed, Halo is the franchise title for the whole platform. Just like Nintendo has Mario, Sega had Sonic, etc. What is Playstation's, Metal Gear? Final Fantasy?

The original xbox was defined by Halo and classified as a 'shooter box'. Microsoft worked hard to shake that stigma because it was hurting sales to casual and family gamers, they went so far to shake that opinion that they didn't release halo with the 360's launch. But that being said, as much as I do like the gears of war universe, i think COD defines the box more. Yes Gears 1 when it launched was like a holy shit moment, graphic fidelity that we've never seen on a console before and it really launched a new genre of 3rd person shooters to a new group of people. I think though that Call of Duty is the game really pushing the system now. With the innovations they've made to shooters including customizations, challenges and rankings, playlists. Yes you saw some of that in othe games before COD did it but they brought it all together in a package the masses wanted to play.

These days I don't think it's a game so much that defines the Xbox 360 as it is a community. Xbox Live is what defines the 360. I know you can have friends on the Wii and on the PS3, but most multiplayer experiences play best on the 360. I'm pretty sure this site was started as the result of a group of people who played pretty regular on the Xbox and 360.

I may be in the minority, but to me, Gears = meh

That’s a tough question, due in part because there are so many good games, but one stands out above the rest that when I think of this game, I automatically think Xbox 360. In 2006 I finally could afford the Xbox 360 and Xbox Live service and was very excited. I never had Live with the original Xbox, so I played Halo, Halo 2, and Ninja Gaiden just by myself or with a friend. However, when the Xbox 360 came out, I was psyched for Gears of War and Halo 3, when it finally came out. Flip flopping between the two games, I usually would end up playing more Halo 3 than Gears, mainly because all of my friends were shooting the covenant as opposed to killing locusts. Then Call of Duty 4 Modern Warfare came out. Everyone was talking about this game. Boasting about the campaign as well as the multiplayer game play. I never played a COD game except for the first one and I didn’t like it, so being the Halo 3 fan that I was, I never gave COD 4 a chance. When the game finally came out, my brother bought it for me as an early Christmas present. I played it and instantly took it out of the 360 and put Halo 3 back in. Call of Duty 4 sucks, so I thought. Halo 3 kicks ass, are you crazy? My brother insisted I give it another go, so I did and unbeknownst to me, the game just clicked. I was constantly playing with friends and family. I would rush home from work just to get in one or two games before dinner. I never felt that about a game, even with Halo 3. Perhaps people are getting Halo gun shy. There is Halo 3, Halo: ODST, Halo Wars, Halo: Reach, and possibly a Halo 4, so maybe people are just tired of it. Call of Duty has multiple games, but a variant of each one. I agree with redeye, when I think of the old Xbox, I instantly recall the Halo series, but with the Xbox 360, it’s Call of Duty all the way.