I’ve always wondered what are the criteria for selecting the best documentary.  Watching Free Solo (Blu-ray release 2019-7-19) does not really provide general answers for me, but it is so unique that it really deserves its award.  It is about the first human in history to climb El Capitan without rope.  The uniqueness of this documentary is that it is exceedingly clear to the film crew, who are experienced climbers themselves, that there is very significant possibility they may capture his plunge to death on film.  Their mere presence puts pressure on the protagonist who is doing this already life threatening sport.