1. Welfare (Frederick Wiseman)
2. Hold Me Tight, Let Me Go (Kim Longinotto)
3. Night and Fog (Alain Resnais)
4. A Married Couple (Allan King)
5. Warrendale (Allan King)
6. Gimme Shelter (Albert and David Maysles, Charlotte Zwerin)
7. Titicut Follies (Frederick Wiseman)
8. Memorandum (John Spotton & Donald Brittain)
9. Eyes on the Prize (Henry Hampton and Various)
10. Domestic Violence (Frederick Wiseman)
11. Middletown (Peter Davis and Various)
12. The Up Series (Michael Apted, Paul Almond)
13. Divorce Iranian Style (Kim Longinotto)
14. Streetwise (Martin Bell)
15. First Contact (Robin Anderson and Bob Connolly)
16. Harlan County U.S.A. (Barbara Kopple)
17. Hearts and Minds (Peter Davis)
18. Crisis: Behind a Presidential Commitment (Robert Drew)
19. The Battle of Chile (Patricio Guzman)
20. The Thin Blue Line (Errol Morris)
21. A Grin Without a Cat (Chris Marker)
22. Shoah (Claude Lanzmann)
23. Salesman (Albert and David Maysles; Charlotte Zwerin)
24. Hoop Dreams (Steve James)
25. Gaea Girls (Kim Longinotto)
There are many more great docs, but I'll just go with 25.