Halo 2 > Halo 3 > Halo 3 ODST > Halo Reach > Halo CE
Based on campaigns.
Halo CE-I did not like this game very much, but I would have to say the best moment was walking out of the escape pod and seeing a colossal ring arching over you
Halo 2-Exacting revenge on the brutes
Halo 3-Advancing through the Citadel and ending the Human-Covenant war
Halo 3 ODST-Mombasa Streets, walking around an all too familiar human landscape
Halo Reach-Probably the last cutscene in Lone Wolf