North by Northwest, Psycho, Shadow of a Doubt, Rear Window, Frenzy, Vertigo, The 39 Steps. I also find Strangers on a Train to be underrated.
Dr. Strangelove would probably be in my top 10. I'm not a fan of 2001 but it is an important film. I think The Shining is the greatest horror movie ever made (and I've seen a whole lot of them) with many layers.