I'm a huge fan of documentaries. Especially those dealing with my favorite and not so favorite flicks and interests.
A few of my favorites would have to be:
Crumb: I <3 this one the most. His family and social life is so fucking interesting to watch.
King of Kong: EPIC. The struggles between 2 men vying for the high score in Donkey Kong was one of the best movies of last year.
Trekkies: I used to be a "trekkie" back in my junior high days. I had a costume and everything. But these people take it to a whole new level of awesomeness.
The Beginning: The documentary on the Phantom Menace dvd. You get to watch George make every mistake that ended up making that movie kinda horrible. Although I love the excitement that Ewan has. He soon loses it.
The Snowball Effect: The documentary on the Clerks 10th anniversary set. Watched this last night. Totally entertaining and interesting to see how he went from nothing to indie darling.
Word Wars: Go behind the scenes of Scrabble tournaments. I really dug this one for some reason. Some of these people are crazy.
I'll list more as I think of them.