The second movie in the America Lost and Found: The BBS Story boxset is the one that artistically put them on the map. It was the Monkees' money that got the movies made, but it was Easy Rider (1969) that got them legitimacy.
Dennis Hopper directs this classic tale of...freedom? People tell me this movie is about freedom. I'll be honest, given the cultural hold this film seems to have I was surprised at how bleak it is.