Top 5

1. First Contact (1982) - Possibly the best documentary ever made https://www.imdb.com/title/tt0085544/

2. Rivers and Tides (2001) - Beautiful meditation on the art of Andy Goldsworthy, with music by Fred Frith https://www.imdb.com/title/tt0307385/

3. This is the Last Dam Run of Likker I'll Ever Make (2002) - The story of Popcorn Sutton https://www.youtube.com/watch?v=glQjCKAI4gA

4. Alone in the Wilderness (2004) - Mostly original footage and commentary by Richard Proenneke https://www.imdb.com/title/tt0437806/

5. Apollo 11 (2019) - Narration-free documentary on the first moon landing https://www.imdb.com/title/tt8760684/

Bonus picks:

Pirates of Silicon Valley (1999) is a dramatization but an incredibly accurate one https://www.imdb.com/title/tt0168122/

If mockumentaries count, everything by Christopher Guest is obligatory. There's also this short gem on the Centrifuge Brain Project https://www.youtube.com/watch?v=RVeHxUVkW4w

Grizzly Man (2005) is not a mockumentary, but like most Herzog documentaries, it's not exactly straight https://www.imdb.com/title/tt0427312/

Finally, I'm a huge fan of first-nuclear-era documentaries made by the U.S. government, e.g.

Army Nuclear Power Program (1963) https://www.youtube.com/watch?v=HPWDMHH4rY4

SNAP 8 Reactor (1963) https://www.youtube.com/watch?v=82nCRWawfhQ

SNAPSHOT (1964) https://www.youtube.com/watch?v=u9jI1Hdl5Yw

Remote Maintanence of Molten Salt Reactors (?) https://www.youtube.com/watch?v=uHT-w2x6dDg

and many more.