Best/favorite Documentary in the collection?

With the B&N sale going on, I noticed my personal collection has a surprising lack of documentaries in it. Looking to see what people's go to recommendations for docs in the collection. Crumb is definitely on my watch list (picked up from flash sale) and I loved Symbiopsychotaxiplasm though that is more pseudo-documentary but i loved the fly on the wall aspect of film making.

As long as the topic is interesting, I want to hear your opinions! :)