Halo 2 > Halo 3 > Halo 3 ODST > Halo Reach > Halo CE
Based on campaigns.
As I always say on these threads
Halo 2: Campaign was fun, good and fun level designs, good aesthetics, great storytelling and storyline, my favourite Halo
Halo 3: Campaign was fun, fun level designs, great aesthetics, storytelling could do with a little polishing up. My second favourite Halo
Halo Reach: Although it had terrible storytelling and storyline,I found it to be a fun, action packed adventure with cool looking aesthetics. My third favourite
Halo 3: ODST: The gameplay is fun and I like the first and last missions the best. Great aesthetics. I would like it so much better if it wasn't for the fact that I dont really like the storyline. 4th favourite
Halo CE: I like Halo CE the least because I find the gameplay boring and tedious, the level designs terrible, the enemies boring to fight, all the weapons were ridiculously overpowered and unfun, poor storytelling and storyline. Extremely let down by the game that supposedly 'revolutionised gaming.'
[Edited on 01.24.2012 12:59 PM PST]