What is one Documentary that everyone should watch and why?

The Ascent of Man - insight into briliant mind of Jacob Bronowski. In the series of essays Mr Bronowski explores his understanding of human mind in nearly poetic show we found connection of chemistry, physics, anthropology, history, drama, and art. He shows us tens of countries, about 40 during span of entire series, but manages to stay intimate thus inviting to his own home seems natural.