Oh my goodness, I have way too many documentaries that I love. Like... I seriously had a fangirl moment just reading the OP.
Anyways. *Clears throat*
My favourite documentaries tend to usually be by BBC. For example
Walking with Dinosaurs
Walking with Beasts
Walking with Cavemen
Walking with Monsters.
Then there's the Life of Birds and the Life of Mammals by David Attenborough. Pretty awesome stuff!
Travel documentaries by Michael Palin ( yeah, the guy from Monty Python) eg Pole to Pole, Himalaya, etc.
Besides BBC content, there are quite a few more documentaries I love. FLOW (For the Love of Water), Home, Africa's Super Seven, The Revolution will not be Televised. There are a bunch more but I can't think of them at the moment.