First one that came to mind for me!

She was the wildcard for me. I certainly knew who she was, but I was also a 14 year-old boy at the time.

Looking her up on IMDB, Kuffs and Chaplin were pretty high profile movies she did beforehand.

First one I thought of, as well.

That also made me think of this movie narrative chart from XKCD:
(spoilerized for LOTR, Star Wars, and Jurassic Park spoilers)

THE TATTOOED STRANGER (1950) is a great indie police thriller filmed on location in NYC with a cast of New York-based actors who weren't name actors.

SALT OF THE EARTH (1954) was made by blacklisted personnel using Mexican-American union members from New Mexico (I forget what union) and a leading lady recruited from Mexico who was deported by the FBI before shooting was completed.

WILD STYLE (1983) by Charlie Ahearn told a tale of the burgeoning hiphop movement in New York and featured graffiti artists and hiphop performers basically playing themselves.

The Blair Witch Project was the first thing that popped into my head, but the poster who said Clerks is probably more spot-on.

Blast of Silence

Except that the title of the thread says "best" so can't be a Kevin Smith movie by default.

Ballast. I don't think anyone was even an actor.

Actually, I think it was that "Act of Valor" movie.