I have only played the main titles, thus far, although soon I intend to add Reach to this completion. Nonetheless, Halo 2 and it is not even close. I was absolutely addicted to that game, logging well in excess of three thousand hours into online multi-player. Frankly I would not be astonished if it breached four thousand.
Strangely enough my interest in the series waned significantly thereafter. I enjoyed Halo 3 for a time, yet never remotely to the extent I had with Halo 2. The game simply felt somewhat shallow by comparison, despite my inability to explain why. Halo CE I was never attached to since I played Halo for the online experience and had no means to play Halo CE online.