Halo 1 is better, but I enjoyed Halo 2.
The first half of both Halos is the best, then the level design gets really repetitive. If the gameplay weren't there to save things, the later levels wouldn't be worth playing at all. What isn't repetitive is repetitive and crap.
That said, keep in mind I still love Halo 1 and like Halo 2. So... I would say the gameplay makes up for a lot. Anyway, I mostly just replay the first five levels of Halo 1. The levels never go exactly the same way twice, thanks to the great AI.
Halo 1 local multi is MUCH better than Halo 2 MP. Seriously. Halo 2 is all power weapons, spawn memorizing and weapon hoarding. Halo 1 gives everyone the best pistol ever right at the start, so every player has a chance no matter what the other guy is carrying. Also, I love the Plasma Rifle and Halo 2 nerfed it. Bleah.