Hard to say, I can name a few must haves:

Pi
The Shining 1980
Night of the Living Dead (the original 60s version)
Carnival of Souls (the original 60s version)
Deliverance 1972
Bad Boys 1983
The Boys Next Door
Two-lane Blacktop
Easy Rider 1969
Beetlejuice 1988
Requiem For The Dream
Pink Flamingos (warning: not family friendly)