Halo Reach was the culmination of all the work Bungie had done with a focus on the Covenant and it made for a game that I thought had plenty of content and a few unique or improved experiences that some of the other Halo games haven't matched. Gets my vote.
If ODST had its own multiplayer, custom games, and forge it would be up there, but unfortunately it only has a story, a great one at that, but Reach had more to offer.
But for atmossphere & all that, Halo 3: ODST has to be my vote, I loved walking around New Mombasa exploring the abandoned streets, & the levels are quite enjoyed