Halo 3 is outstanding....if you've played the first two. If you're just getting into the series, I don't think you'd get the same experience out of it. It is one of my all-time favorites, but that is because it wrapped up a tremendous storyline. They just kept getting better and etter.
Reach is an outstanding game, and packed with a ton of things to do. Campaign, Multiplayer, Firefight, Forge, Theater. And even though it's currently the newest Halo game, it's a very good place to start if you're new to the series.
ODST, was my least favorite of all the Halo games....and that includes Halo Wars.
I didn't care for the city. Nice idea, but you really couldn't interact with it much.
I'm one who didn't care for the voice actors AT ALL. Thought the actors, combined with the goofy dialouge, was a huge letdown. And I know some people loved the music, but that was something else that just ruined the game for me.