I mainly play ODST and Reach, simply because I loved the story and gameplay in ODST, and Reach's Multiplayer was an absolute blast.
I still play Halo Reach out of all of these the most. It has daily and weekly challenges, and its the newest out of all of the games on this poll. I don't play the other games very much because I have MCC for that. Still, sometimes I do go back to the original games just to see how Halo started.