Halo and Half-Life are the only two FPS franchises I can get at all excited about. Pretty much every other FPS out there is just too grey, dull, uninspired and boring for me to get past the first 60 minutes. (Unless we count MP series too, although perhaps they're technically FPA).
What I love most about Halo is the atmosphere, it just totally draws you in to its world. Also, the superb AI just makes the gameplay far more fun, even on Normal. The attention to detail, colourful and varied environments, music and feeling of being a tiny part of something epic is great. I'm not even talking about the multiplayer.
Halo isn't perfect. It has flaws like everything else that's beautiful.