I know most people are going to vehemently disagree with me on this, but ODST for me. I am not a multiplayer man. I have never once played XBox Live with Halo unless it was at a friend's house, because I do not have XBox live. The only game I really enjoy the multiplayer of is TF2, which gives me no other choice BUT multiplayer. Therefore, the shamefulness of the multiplayer in ODST is only a very minor factor. I want story. I've ready most of the Halo novels, and loved them. I've played through all the single-player campaigns, and loved them. The moment I saw ODST in the title of the game, I knew I was in for something very different story-wise. Something more akin to the novels, which I honestly thought were far superior to the games. The story did not disappoint. For me, the story was everything I was hoping for. Get to be an ODST, the most bad-ass non-Spartan people in the game? Check. Have a much darker, more intense story? Check. Although you get some emotional connection to Chief and for some even the Arbiter (most people who were Player 2 for Halo 3 like the Arbiter more than those that were Player 1 for obvious reasons), the connection to the guys in ODST is surprisingly strong for the 4th game the franchise has put out. Usually introducing new characters at that point is frowned upon, let alone basically saying "Fuck the original cast."
I haven't gotten my hands on Reach yet, so I can't say if that's better or worse. Everyone's saying better, though, so I can't wait 'til I do.