Halo 2 > Halo 3 > Halo 3 ODST > Halo Reach > Halo CE
Based on campaigns.
Halo 2 because the campaign is fantastic and the it really drags you into the story
Halo 3 is my second favourite. The campaign has a great story and fun to play, although the actual storytelling could be improved
Then Halo Reach, the story is crap but the campaign is fun to play
Then ODST. I think it has great storytelling and fun gameplay but I don't like the half arsed attempt to link the storyline with the main story of the trilogy at the end
Lastly CE (zips up flame suit) because I think the storytelling is terrible and the storyline had so much potential. I hate the level designs and the boring repetitive enemies, plus the unfun weapons and vehicles.