I'm not a major fan of documentary TV, mainly because I like to have fun instead of learn things when I'm tired. That said, there's a few series that I enjoy.
David Attenborough (I can watch a lot of things with him in it)
Deadly 60
Fierce*
Idiot Abroad, An*
Love on the Spectrum*
Moaning of Life, The*
Planets, The (1999 BBC Series)*
TravelMan*
Walking with Dinosaurs*
You Can't Ask That*