I played Halo: Reach, Halo 3: ODST, and Black Ops over Christmas break. I would say that Reach had the most intense gameplay and I found it harder than Halo three. I really enjoyed firefight mode on it. I thought Black Ops had the most epic story; my draw had literally dropped by the time I completed the campaign. Very cool and original story.
ODST was the most fun to play out of the three. It just might be my favorite halo because it wasn't trying to tell an epic story and was just a fun fps with more humor than any of the other halos. It also had a sense of comraderie and teamwork, something I found lacking in the other halos.