In my opinion its between halo 2 and 3 but if I had to pick one I would have to say Halo 3. I would say Halo 3 because for me it was more exciting. Specially the campaigns, the skulls, the achievements, forge, theater, ect. That was brought newly to Halo and it seemed very fresh and exciting. Also because in my opinion the Halo 2 Campaign was just extremely easy. But the maps in halo 2 were pretty much better than Halo 3. Except for Guardian which is my favorite map in Halo 3.
Its my opinion. What do you think?