By the power of truth, I, while living, have conquered the universe.
ODST's story was nothing short of perfection.
It's a much deeper, darker and more personal story than Halo 3 ever could have been. The sense of being alone in a fragmented city in the middle of night whilst searching for your scattered squad was a fantastic new frontier on Halo's campaign design and was pulled off wonderfully.
Then there's Sadie's Story, in my opinion it's one of the best things that's ever happened to Halo. Everything from the voice actors to the art was stunning, the story itself was more emotional than anything else in the other games because we were seeing this all from the eyes of a civilian instead of a battle-hardened warrior who doesn't really give a flying toss or show emotion. The inspiration drawn from Dante's Inferno was certainly something exciting and new.
Then there's the music, one of the finest pieces of work Marty has accomplished. The sax solos, guitar riffs and piano segments all fitted the game like a glove.
Halo 3 was one of the biggest disappointmets in the series, second to Reach of course. We had this incredible build-up from Halo CE and Halo 2, then we were left with a lackluster conclusion which proved to be too much of a "deus ex machina" ending. Yes, it had its good points like the death of Truth and fighting two Scarabs at once but otherwise the mission design was too dull and had a lot of backtracking in it.
ODST takes the cake for me.