I've been watching (and enjoying)
Current's series hosted by documentarian Morgan Spurlock.
Part 1 (50-41)
Spellbound (Jeff Blitz, 2002)
Truth or Dare (Alex Keshishian and Mark Aldo Miceli, 1991)
The Kid Stays in the Picture (Nanette Burstein and Brett Morgan, 2002)
One Day in September (Kevin Macdonald, 1999)
Little Dieter Needs to Fly (Werner Herzog, 1998)
The Decline of Western Civilization (Penelope Spheeris, 1981)
Burma VJ (Anders Østergaard, 2008)
When the Levees Broke: A Requiem in Four Acts (Spike Lee, 2006)
Catfish (Henry Joost and Ariel Schulman, 2010)
King of Kong (Seth Gordon, 2007)
Part 2 (40-31)
When We Were Kings (Leon Gast, 1996)
Biggie and Tupac (Nick BroomField, 2002)
March of the Penguins (Luc Jacquet, 2005)
Inside Job (Charles Ferguson, 2010)
Taxi to the Dark Side (Alex Gibney, 2007)
Paragraph 175 (Rob Epstein and Jeffrey Friedman, 2000)
Brother's Keeper (Joe Berlinger and Bruce Sinofsky, 1992)
Tongues Untied (Marlon Riggs, 1989)
Dogtown and Z-Boys (Stacy Peralta, 2001)
Jesus Camp (Heidi Ewing and Rachel Grady, 2006)
Part 3 (30-21)
Farenheit 9/11 (Michael Moore, 2004)
Man on Wire (James Marsh, 2008)
Gasland (Josh Fox, 2010)
Tarnation (Jonathan Caouette, 2003)
Murderball (Henry Alex Rubin and Dana Adam Shapiro, 2005)
Enron: The Smartest Guys in the Room (Alex Gibney, 2005)
Paradise Lost (Joe Berlinger and Bruce Sinofsky, 1996)
The Eyes of Tammy Faye (Fenton Bailey and Randy Barbato, 2000)
Shut Up and Sing (Barbara Kopple and Cecilia Peck, 2006)
Exit Through the Gift Shop (Banksy, 2010)
Part 4 (20-11)
Capturing the Friedmans (Alex Jarecki, 2003)
Touching the Void (Kevin MacDonald, 2003)
Food, Inc. (Robert Kenner, 2008)
Street Fight (Marshall Curry, 2005)
Bus 174 (José Padilha and Felipe Lacerda, 2002)
Crumb (Terry Zwighoff, 1994)
Dark Days (Marc Singer, 2000)
The Fog of War (Errol Morris, 2003)
Bowling for Columbine (Michael Moore, 2002)
Paris is Burning (Jennie Livingston, 1990)
Part 5 (10-1) will air August 31, 2011
My guess is that the following will be among the top 10:
The Thin Blue Line (Errol Morris, 1988)
Super Size Me (Morgan Spurlock, 2004 )
Hoop Dreams (Steve James, 1994)
Grizzly Man (Werner Herzog, 2005)
The Times of Harvey Milk (Rob Epstein, 1984)
An Inconvenient Truth (Davis Guggenheim, 2006)
Waltz with Bashir (Ari Folman, 2008)
All of the documentaries on the list were made in the last 25 years.
The Up Series (Paul Almond and Michael Apted) spans 49 years, so it won't make the list. It's a commitment to watch all seven episodes, but as a whole it is incredibly fascinating.