The Wild One (1953)

Directed by Laslo Benedek

Loosely based on a real life story of a biker gang terrorizing a small town.  Brando is just effortlessly cool here.  One of the first "counter-culture" movies, this movie basically solidified the image of bikers in pop culture.  It's difficult to overstate the influence of this movie on countless later sub-cultures like rock'n'roll, and punk rock.