Halo reach is by far my favorite and most "complete" feeling experience. Tons of game modes for multiplayer, a great single player with some interesting characters.
Enough has been said about Halo CE, a great bonus is the game comes with a 7 map dlc code for Reach. But if you haven't played Halo CE, this is an amazing upgrade to one of my favorite campaigns of all time.
ODST is great for a little expansion, if you look at it like that. If you are a fan of Firefly, having three of the voice actors doing their firefly personas is quite a nice touch.
I personally feel 3 was the worst in the series, the campaign was quite straightforward and the multplayer level designs had a lot of problems. And at this point you need all the DLC to play 90% of the playlists online, so be ready to buy.
*The Cadillac of signatures*