HL by far.
Halo's repetitive levels have become something of a cliche, but things don't become cliche without being true. And as if the repetition wasn't bad enough, the game actually has you retracing your path through levels you've already truged through.
Halo's level design is the worst since Legend of Kage (NES). Ok, that's an exaggeration, but I honestly haven't seen worse for a long, long time.
If every level were on par with the very first level when you escape the Autumn this could have been the greatest FPS ever.
If this and nothing else is cleaned up in Halo 2, it could be worthy of the praise that the original received.
My next complaints are personal preference. First, aliens have ruined every "story-based" FPS I've ever played. PD, even the end of Half-Life was destroyed when you went to the alien planet. The only time they haven't ruined the experience for me are in games like DOOM and Serious Sam. Again, just a preference.
I also didn't like the vehicles. *gasps* The only vehicle that I enjoyed was that giant, nearly indestructable Tank that you could drive for about 100 yards. The flying vehicles were too difficult to control and aim with and the Warthog's physics were a big turn off.