What makes a good documentary?

Last year there were two big animal documentaries The March of the Penguins and Grizzlyman. Both about animals but they couldn't have been different. And I probably already made a mistake since while The March was actually about animals Grizzlyman was much more about the main character and about people rather than the animals. I personally liked both movies, but Grizzlyman did touch me more and I would give it at least two stars more.

I liked both movies. I really like the one about certain people in history.