Since the Academy of Motion Pictures has changed the definition of what "documentary" means, I think we need to re-visit some older films. Since films that are mostly fictitious now qualify as a "documentary", there are many past films that I feel should get retroactive Oscars for being excellent documentary films.
The Godfather: This is an excellent "documentary" on immigration to America, particularly from the Italian point of view.
Jaws: Who can forget this fascinating "documentary" on marine biology?
Star Wars: This is a very in-depth look at the American Space Program. Many NASA scientists give their points of view. I particularly appreciate the disagreement among scientists as to the nature of the force. This "documentary" gives you a lot to ponder.
Sideways: Alexander Payne's fascinating "documentary" that explores Southern California's coastal wine region and delves into how wine characteristics reflect on those who drink them.
Cinderella: This "documentary" examines the conflict between the ruling class, the middle class, and the lower class. It's particularly effective at the end where it shows it is truly possible for someone in the lowest class (a slave) can actually rise up to the ruling class with a little luck.