Pretty much the only FPS I've played
HALO: CE gave me one thing: A STORY. I don't play games to play games. I want a *reason* I'm in there putting holes in aliens. HALO gave me that, and gave me a reason to shoot those little bastards. It also gave me a reason to slog through the library. It kept me engaged.
As a game, my reflexes suck, so I don't do multiplayer, but I still want a decent challenge from the AI. HALO gave me that too. It also gave me a HUGE sandbox, with enough varied stuff I could keep trying different things. All the physics worked and there was no "rails" to ride on. You could try carrying different weapons, and sneaking instead of frontal assaults, and other things.
It even had controllers large enough to fit my paws, until they went to the small ones, but I found a bunch of large ones on eBay.
For some reason, I loved the one where you played as a marine, with the cool heads up display and finding the audio tapes. I can't remember the name, and nobody else liked it, but I did.
And as an added shock, local LAN multiplayer was actually fun with my friends, so we played that for a LONG time. We never did XBOX Live. I did for a short while on Halo: Reach, until I got banned for "cheating" because my ISP was so shitty it kept dropping my connection. When that happened, the XBOX went on a shelf and has stayed there.