In my opinion, most of the Halo series was great, and I had lots of fun playing the games.
Halo: Combat Evolved gave me a warm welcome. The controls were neat, the gameplay was good, and my expectations were exceeded.
Halo 2 re-invited me into the sci-fi world with new equipment, new vehicles, and more. Quite fun, will be happy to play it again.
Halo 3 felt like a fitting end to the Halo series. Main characters from the previous games die and there's new stuff put into the game. However, some things from the previous games have been removed as well. Overall, it was pretty good, and the gameplay was refreshing.
Those are the games I thought were good. Halo 3: ODST, Halo Wars, and Halo Reach were rotting piles of stuff. That's my opinion, anyways.