It's a tough choice, but here goes...
Debbie Does Dallas...... oh, you don't mean THAT kind of film?
OK, seriously.... Goodfellas would definitely be up there, but these are my three.
1) Memento (a film backwards?)
2) The Beyond (Fulci at his very best)
3) The Seventh Seal (Bergman shows the wannabes how to make a great film) Ugh... subtitles? I wanna watch a film not read it!!
Monkey