I'm a multiplayer guy, so I always like the game with the most active community at the time (Reach, basically.)
Halo 1: I felt like a REAL super soldier, and killing stuff always felt awesome.(CEA reflects this nicely.)
Halo 2: Best multiplayer. Playing multiplayer maps there now...is very eerie and alone, a shell of its former self.
Halo 3: Good multiplayer and great maps. It has one of my favorite campaigns.
Halo 3 ODST: Firefight was the AMAZING(I could go paragraphs into why its amazing). Reach's firefight was terrible compared to this. Campaign was good, but the open feeling of Mombasa Streets was really fun. Too bad about the lack of MM.
Halo Reach: The only Halo where I've developed pet peeves. I dislike armor lock, but the banshee bomb's AoF has made me want to ragequit.