Arnold may be one of the best embodiments of a movie star. It would be insulting to say he doesn't or can't act. He does! It's just he's best at being "Arnold." That's a limited range, but it's something he's great at.
That's the advantage that he has always had over his rivals in the Great Planet Hollywood Box Office Wars of the 1980s and 1990s. He knows his limitations and has always been shrewd at working within them. He's made bad movies, but he doesn't miss the way others miss.
Sylvester Stallone and Bruce Willis are undeniably better actors. When they're on, they can be great. But they're also not as good of actors as they think they are. Sly, god love him, just ain't funny and Bruce somewhere along the way lost his charisma. Doesn't/didn't stop either from trying!
Arnold, however, has always keenly understood that he is "Arnold."
So really, in a lot of ways, Pumping Iron may be the best Arnold. Because he's been Arnold all along and forever more.