I would offer you to take your pick. Moore has been divergent to an extent in his film-making. His early films such as "Roger and Me" and "The Big One" took major and direct aim at unfair business practices and struck a very populist chord in its assertion that wealthy industrialists are only that because of the economic brutality perpetrated upon their workers. His work took a more decidedly political, anti- Republican stand with "Bowling for Columbine," "Fahrenheit 9/11" and "Sicko." These films presented a very strong bias against the practices and beliefs of the Republicans, and in particular, former President George W. Bush. "Capitalism: A Love Story," released in 2009 seems to combine both his populist message and his strong biases against the Republican party.
Well, Michael Moore's films are a good place to start, and if you're looking for bias, then Fahrenheit 9/11 is probably his most slanted work. Moore was no fan of the Bush Administration, and the "facts" he uses in the film take some giant leaps and assumptions to make them sound like credible arguments. In almost all of his films, he overuses emotional appeals to win over the audience, so much so that it's obvious. While the points he makes aren't necessarily always wrong, his filmmaking subjects and techniques, and overdone narration would definitely put him in the "biased" category of filmmakers.
Every documentary has some bias. Even the new documentary, "The Art of the Steal" is biased for the Barnes family. You can not have a complete unbiased documentary even if it was all news footage, because one can edit it with any type of message.
In "Fahrenheit 9/11", what are the messages (the invited reading) that Michael Moore tries to promote to the audience?
e.g. Repbulicans are bad, war is unecessary, ect.