Anyway, Halo takes the cake, or I probably wouldn't be on these boards.
I like Halo because of its fun, and simple gameplay. Halo also made the xbox what it is today, and had some interesting gameplay mechanics which most shooters use today, such as limiting you to two weapons, separate grenade throwing button, vehicular combat within a 1st person shooter, etc. Also, without it, CoD probably wouldn't be as popular today, considering most people play it on the 360, and Halo gave the xbox its popularity.
Anyway, on to some more actual reasons; the story is really good, especially for a 1st person shooter. It takes you to interesting places, and not to just some place that looks the same as another. The music is amazing as well. Marty O'Donnell just makes the best music, and its always on the right time. Halo 4's soundtrack is pretty good too, but no one can compare to Marty's soundtracks. Thats why i felt bad for Neil Davidge, i mean, how do you outdo something already amazing?
Now, thats not to say that Halo is flawless, but i can't think of anything too major, other then maybe a few gripes, like how some weapons suck, and are pretty much the exact same weapon, but for the opposing side. *looks at Storm Rifle*
But i really don't know why people compare CoD and Halo so much. The multiplayers are pretty damn different from one another.
CoD's multiplayer has more of a lone-wolf, Rambo mentality to it, which is why i think people like it so much, is because you get the killstreaks, and feel unstoppable. Its got the customization, perks, killsteaks, etc.
Halo's multiplayer is more team-based, where if people on the other teams are communicating, and your team isn't, your going to get steam-rolled. Halo's is more arena based, where everyone pretty much has the same exact things, and the power weapons are scattered along maps, which is nice, because it creates natural battlefronts where everyone is fighting at, since everyone wants the power weapon.
In the end, its all a matter of opinion.