By the power of truth, I, while living, have conquered the universe.
ODST.
Reach was a mandated project, you can really tell that it wasn't designed with the same sort of longevity Halo 3 had in mind because Bungie is just tired of the Halo series.
Reach had an awful story that completely defied the past 9 years of canon, it was a conglomeration of poorly crafted fan-service that stuck to generic archetypal characters and cut out the heart of what made Halo fun.
I enjoy matchmaking, but I feel kinda cheated that the maps were simply pulled from the campaign. It's not original at all, it's boring, Halo 2 and 3 had absolutely awesome spaces for multiplayer because they were designed with multiplayer in mind from the start. Since Bungie always starts with the campaign in their games, the multiplayer spaces were simply adapted from single player environments.
Forge has a grand total of 2 good maps for the feature. Forge had so much potential in Reach, in fact the new features live up to the premise, but the lack of decent maps is a let down.
Firefight is less fun in Reach too. Gone is the sense of desperation as you spend hours trying to pass that 4th set on Heroic, it was majorly toned down. Being able to customize Firefight is great, but you can't play your own custom games in Matchmaking so you have to rely on your friends list.
ODST had a great plot, it was a nice change from the trilogy to have a more laid back story with a darker theme and side stories that managed to come to a great conclusion with the audio files. ODST just felt like it had more to it, the characters were still generic but they were miles better than Noble Team.
ODST takes the cake.