My fav is probably my Mein Liebster Fiend, about the relationship between Werner Herzog and Klaus Kinski, who made several legendary films together. Kinski was a German-level madman but a really fascinating actor. I'm a big fan of films like Aguirre: The Wrath of God and Fitzcarraldo where Kinski starred and Herzog directed
Another really cool one on this same topic is Burden of Dreams, which is about the making of Fitzcarraldo. The central gimmick of which is dragging a steamboat over a hill in the middle of the Peruvian rainforest. They worked with local tribes and a funny story is that they were so shocked by Kinski's violent outbursts they offered to kill him for Herzog
Then on a similar lines of making movies in difficult terrain and circumstances, Hearts of Darkness, about the making of Apocalypse Now