Bowling for Columbine, Inconvenient Truth, A Funny Thing Happened on the Way to the Moon, etc. would convince no one if they were being subjective. Anybody who looks into Michael Moore's anachronistic and manipulative editing methods, digs a little deeper on the GW thing, or does a tiny bit of investigation into the moon landings, the materials brought back, or checks up on the guy who is always pushing the fake moonlandings and his motivation for it, they are going to have a completely different opinion about any of those subjects than they would if they watched the documentary and walked away feeling they know everything there is to know on the subject (which, unfortunately, is what most people tend to do when viewing documentaries).
Documentaries by their very nature are bias. For a good example of this, watch Empire of Dreams then read Zombie's book, you'll see that something as simple as a documentary on the making of a series of films can still be extremely biased.