I think Halo 3's campaign was the best of the three, though it was too short and I wanted so much more. There were hardly any annoying moments where I had to do the sameover and over again, and I had a decent challenge without getting stuck very often. Extremely fun on Heroic and Legendary. 4 player co-op with team scoring and skulls = massive win. I've played through campaign about 2.5 times, now.
Also, you haven't lived until you've Rocket Raced.