Chad Feldheimer, 'Burn After Reading' (2008)
There are a lot of outrageously moronic dumb-asses on this list — but Chad is a frighteningly real idiot, which somehow makes him more terrifying than your average screen doofus. Idiots are a Joel and Ethan Coen staple, but while only appearing in half the film, Brad Pitt's fitness-obsessed, gum-chewing gym rat solidifies himself as the best/worst of the brothers' braindead bunch. Chad's true colors shine when he picks up a disc of confidential data. Or, he thinks it's confidential, because long lists of numbers and dates must be top secret government files that are all long lists of numbers and dates. Obviously. His blackmail plot is fit for a seven-year-old, complete with victory dances (performed alone and for present company), a "serious" voice, and a well-oiled plan that boils down to "give us money?" When he's caught in his lie, Chad makes a great point: "Appearances can be... deceptive."
Dumb Factor: 7