Halo 3 by far. I'll probably be flamed for this. However, it is my opinion either way.
Halo 1 was a true science fiction masterpiece. Absolute must-own for all owners of Xbox, Xbox 360, or a relatively modern Windows PC worldwide. Regardless of genre interest.
Halo 2 was meh. Worth renting, but didn't have that hard to describe "spark" great games do.
Halo 3 is outright terrible. Poor level layouts. Horrid jaggies when upscaled. Boring multiplayer, the act of firing a weapon just didn't satisfy like it should. I find even barber shops more enjoyable than this garbage.