I'm not a body building afficionado but I think Pumping Iron is an absolute classic.
As for Arnold, surely he took bodybuilding to a new level. No matter how many contests Haney, Yates, Coleman or whoever else wins, there will always pre and post Arnold.
Partly it was good genes. Look how long his limbs are and how fine his joints are. He was obviously a perfectionist, able to work every inch out of his musculature. By comparison his competition looked underdone even if they were bigger (Lou), more muscular (Franco) or more psycho (Katz).