Halo, by far. While Halo 2 is definitely more balanced as far as weapons go, it lacks the "feel" of the original. The main contributor of this is most likely the difference in level design. The original had wide-open vistas, while giving you confined indoor areas in between. Halo 2 has greatly expanded levels, but perhaps too much so. The indoor levels are more freeform, and allow for more tactical options, but the majority of the outdoor areas are too wide-open, and simply allow you too many options. Not to mention the vehicle sequences, which always end up becoming Ghost/Banshee target ranges.
I preferred Halo 2, simply becuase melee attacks feel much more satisfying, the squad AI is a whole lot better and you now have weapons that reward accuracy - ie the battle rifle.
Bungie tried to change too many things with the second halo thus not sticking to halo's roots. It didnt feel like halo to me. Halo 2 was ok but far from the perfection of halo.