Top of my list would be "Dancing Outlaw" the story of Jessco H. White, an unconventional free spirit from West Virginia, and his alternate personalities "Jesse" and "Elvis".
Right next to it would be "The Cruise" the life and times of New York tour guide Timothy "Speed" Levitch (featured in the cult classic "Waking Life").
"Hoop Dreams" is considered the gold standard for documentaries --the story of two inner city kids watching their dreams of NBA stardom fall apart.
"Paris is Burning" - the complex lives of New York city drag queens in the 80's.
I can also recommend an entry from last year's Film Fest (one of my wife's close friends was a producer of the film) "Which Way Home", about two street kids sneaking illegally into America.
Last years' slam poetry doc "Louder than a Bomb" was great. Along a similar line was "Rize", about the development of a new form of street dancing in California.
I've got more, but these are good places to start. Snarf, you'd love "Outlaw" if you haven't seen it yet. Jessco is also featured in two other documentaries, including last year's "The Wild and Wonderful Whites of West Virginia"
OH --and of course for local flavor there's the award-winning but controversial depiction of gentrification tensions in Old Towne East -- "Flag Wars"