Halo 1.... why?
well let's look at this, Halo 2 was a Bitter disappointment, in halo 3 it was ok but the online mode was so extremely bugged and the campaign was waaay to easy, ODST never existed because if it did i'd have to disown halo all together, Wars was fun, but would be better on PC, Reach was just about getting the most honourable death with a slightly different multiplayer to halo 3.
so yeh, Halo 1... i still play it today (occassionally), mainly just campaign, because, 1 you can mod the campaign maps, 2 the campaign isnt easier than pie to complete, 3 the multiplayer also can be modded to boost user enjoyment, which is the only place where i play halo.