I'm late to this party buuuut:
-All of the BBC's "Planet Earth" are amazing.
-For the hell of modern warfare: "Restrepo"
-Historical stuff: "The Fog of War."
-Holy shit, that's messed up: "The Act of Killing"
-"The World at War" is the best World War II doc out there, probably. Was made in the 70's so there are a lot of interviews with people who lived through it and who were still, you know, I don't wanna be offensive but like, they weren't remembering things that happened 60 years earlier. It was a fresher memory being recalled by more correct minds. Really fascinating. It's like 20 parts though, so don't expect to finish it in one day or even one week.