What I loved about it is the story and the gameplay. Call it glitchy, call it unfinished, call it what you will. Fact of the matter is that the glitches in the campaign weren't game killers, and in fact HELPED the player more often than hurt them. I remember on a certain level playing with my friend (Jackals, in the courtyard! lol) we were trying to decide whether or not to go AROUND the bad guys. Games don't usually give you the option to completely sidestep enemies. Especially in a linear game. Even though it was unintentional, it was still fun. Personally, it made me feel like a friggin ninja. And the dialogue. Oh the dialogue was a riot. It was the only Halo game that I literally had to pause because I was laughing so hard. That's what sold it for me.
For me campaign story best to least, it's Halo 2, Halo 3, Halo CE, Halo ODST, Halo Wars, and Halo Reach.
For campaign gameplay, it's Halo 2, Halo CE, Halo 3, Halo ODST, Halo Reach, and Halo Wars.