Aish..that's tough to answer. I guess you can say it depends on your genre taste.
I for one love watching documentaries that cover: Space, The Universe, Crime, Egypt, Aliens and Wildlife. But I don't like watching documentaries on Ocean wildlife and don't really enjoy technological documentaries like Mythbusters.
And if you do love watching the same genres as I do, I still can't answer because every single doc I've watched on them are superb!
Still though, you can go on the Web and check out 'National Geographic's' website. They have everything. I think :lol