True crime. Politics. Religion. Biographical. And often controversial.
I love documentaries. Many have a touch of bias to them, but the best ones don't bother you with the bias. They educate you through it. If the director didn't have an agenda (i.e. ax to grind), they wouldn't have driven to make their films.
P.S. - You'll notice I like films about films and what goes into making good comedy. :-)