In the broadest categories: Science, History, and 'personal interest' are very cool as well.
I love WWII documentaries, too.
Like WWII, other history, nature, wildlife, to name a few.
wildlife and nature
Animal and wildlife and science
Anything by David Attenborough, and those focusing on natural disasters past and present.
house remodeling :D
The ones that start with a British guy in a field talking.