Halo 2 > Halo 3 > Halo 3 ODST > Halo Reach > Halo CE
Based on campaigns.
As i always say in these threads
Halo 2-The best, fun to play, storytelling and story is fantastic, fun weapons and vehicles, its great!
Halo 3-Great gameplay and great story, but 5 missions heading towards the portal is a weakness
Halo Reach-Storyline and storytelling suck major balls, but its a really fun game to play through, especially on co-op
Halo 3 ODST: I loved it until the crappy half assed ending, still really fun to play
Halo CE: Boring level designs, enemies way too easy, OP weapons are unfun to use, repetetive, crappy story, crappy Flood, boring. Thank god it wasnt my first Halo game it would have probably put me off Halo for life