Eraserhead. I've actually watched it multiple times (it's my favorite David Lynch film), but I wish I hadn't. I wish I had only watched it once, and let it become more and more dreamlike in my memory. Now I feel like I know it too well, and familiarity removes some of what made it so great: the disorientation and vague sense of fear and disgust.
Same goes for Jan Svankmajer's Alice.
Came here to say Eraserhead... god, what a film.