I'd have to go with ODST.
First, ODST had the most brilliant campaign maps I have ever seen. The city was beautiful, and it has many secrets locked away inside it. NMPD? Fighting on Skyscrapers while an amazing skybox looms ahead? Awesome.
Second, Gameplay. I don't know if it's just me, but I feel that the AI in ODST is better then Reach's. And ODST wasn't just run and gun. You had to think about who you attack. I liked that better.
Third, Story. Great personalities, tension between the team, it all adds up. This story is neck and neck with CE in my opinion. Amazing story.
Fourth, Sadies Story. Best thing that's ever happened to Halo.
This is the most I've ever been interested in a video game. I wanted to find those audiologs SOOOO much. I wanted to know how the story played out.
Fifth, graphics. Turn off Visr for a bit in the city. THe lighting is just amazing.
These are in no order of significance, just the order I chose to write them in.
In my opinion, ODST wins by far.