Ask HN: What are some of your favorite documentaries?

This question has been asked several times before (see below), but looks like the most recent was about a year ago.

Thoughts on why are also appreciated.

1 - https://news.ycombinator.com/item?id=18085765 2 - https://news.ycombinator.com/item?id=18537512 3 - https://news.ycombinator.com/item?id=18271167 4 - https://news.ycombinator.com/item?id=32799789 5 - https://news.ycombinator.com/item?id=41906454