Halo 2 > Halo 3 > Halo 3 ODST > Halo Reach > Halo CE
Based on campaigns.
Halo 2. I judge on campaigns, and Halo 2 has by far the best campaign, the story and storytelling is great, the gameplay is fun, the weapons, vehicles and enemies are great, the missions are great, its just a fantastic game.
IMO
Halo 2 > Halo 3 > Halo 3 ODST > Halo Reach > Halo CE
Thats my opinion on the Halo campaigns.
And to those about to ask me why i put the 'revolutionary classic' last. To those of you who may have seen my CE hate threads a while back, by harsh feelings towards the game have calmed down. There are a few things which i like about this game, such as the needler, the SC and AotCR missions and the melee animations. However, the fact that none of the other missions have replay quality, the weapons are IMO overpowered and the enemies tedious to fight, make this my least favourite Halo