For me its the diversity and beauty of the maps. Halo is such a rich universe that the maps can be in any terrain and in any building. Something I've only really found else where in Unreal Tournament. GoW maps are dark and depressing and CoD are all set on earth, so they feel one dimensional.
How bout you? Maybe its the guns, maybe its the game play, I don't know you tell me.
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It's unique story and gameplay.