Exit Through the Gift Shop - not only the best documentary of 2010, but probably the best movie too. I don't care if it's an elaborate hoax or not (it would have to be a massive one, but that's why people get suspicious about it, because it's so freaking flawless), it's still the most amazing tale on how trends work in popular culture, from being subversive to commercialism to self-parody.
Man on Wire, King of Kong, Anvil, Overnight, Grizzly Man, some of the best documentaries of the past years. Exit tops them all. See it.