[SmG Facundo]

The hardest based on general public opinion and review sites is either Reach or Halo 2. (Halo 4 campaign wasnt accounted for)
I find Halo 2 Campaign even on Heroic much MUCH harder than Halo 1 Campaign on Legendary.
Agreed though, halo 1 campaign was a really great experience.

Nah, Halo Reach campaign was the easiest for me. I haven't completed on legendary yet (I'm on mission 8) because my disc broke up like 4 months ago... but the progress is really fast. The lack of floods made it much easier.