Academy Award listings.. Not necessarily the best documentaries, but it's a good start.
Grizzly Man was very good. About some guy that thought he could go up and hang out with wild bears and not get hurt.
The battle over citizen kane was also very interesting.
The recent Man on Wire was also excellent.
Whether or not you'll watch (and/or enjoy) a film by Michael Moore is dependent on your world view, so you'll either embrace it or reject it. Up to you.
I really liked Planet Earth.
Then there's Touching the Void, Tokyo Noise, Surplus, Project Grizzly, Helvetica (new film about the font, really good!), Who killed the electric car.
Smile Pinki was good - I think it won an Oscar a year or two ago. It's about the work of the Smile Train organization, a non-profit that arranges for surgeries to correct cleft lips. In Smile Pinki it shows a few of the different kids in rural India that received the surgeries.