1. Halo 2 - I felt this had the most varied environments, and the set pieces were wonderful. The first time I blew up that Scarab on the Earth level was so orgasmic, few games could provide a more satisfying feeling. I think the storyline in this one was also the most cinematic and interesting.
2. Halo CE - A classic. Also has great environments, a great story and by far the BEST weapon sound effects EVER IN A GAME. I still wonder to this day why the rest of the Halos never brought back the original Assault Rifle sound effect.
3. Halo 4 - A very short adventure, but the interaction between Chief and Cortana was very emotional, and goddamn that flying level is probably #2 in greatest Halo moments, following the Scarab in H2. I also was a bit disappointed there were only two new enemy types.
4. Halo 3/Reach: I do really like both campaigns but have some problems with both as well. Halo 3 had too little enemy variety and the environments got a bit stale after a while. Reach differs in that it has great enemy variety and really great environments, but the characters are simply unsympathetic, uninteresting, and the writing for it was generally weak.
Didn't play 2's campaign. Yes, I do think Reach's campaign was pretty awesome. Yes, I also thought the game in general was pretty terrible, I'm not a Reach lover, though I gotta admit, Halo 4 is my favorite Halo since CE.