Halo 2 > Halo 3 > Halo 3 ODST > Halo Reach > Halo CE
Based on campaigns.
Bungie is no longer managing the Halo Universe. While our Halo-related forums and stats remain available, be advised that "official" answers and support about the Halo Universe (past, present, or future) are not.
Also, why don't you just buy a copy here, it's cheap now