Five Easy Pieces (Bob Rafelson, 1970)
In his first - and, very possibly, still strongest - lead, Jack Nicholson at first appears to be playing a moody blue collar worker itching to ditch a dumb girlfriend he doesn't love. Through terse yet textured, consistently ingenious vignettes, you gradually get to glean an unwieldy personal history and an equally unwieldy mind that has contributed towards his unsteady handle on things like responsibility and contentment.
The subject matter - the oppressive bourgeoisie pushing one of its own into uncertain drifterhood - is very much tied to the early 70s zeitgeist. But the film's timelessness comes from the maturity and complexity with which it is explored. It remains a strong influence on today's moody independent filmmakers, conscious or otherwise.