Ice Pirates!

A  film called The Irrefutable Truth About Demons from New Zealand I think.  It keeps showing on Showtime Beyond.  I love it - I think it's a great example of how you can use pretty simple make-up and stuff to make people seem really crazy.   I can't really explain why, but I really like the leads (Karl Urban, Katie Wolfe and Jonathon Hendry).

Invasion of the Body Snatchers (the original).
Rosemary's Baby
Stepford Wives (the original)
Ju-On (the version before the major American release, which ended with a shot of a Tokyo empty of people and full of missing notices)
Krull (for the cool weapon and the Cyclops)
The Man Who Fell to Earth
Just about any 70s vampire movie, most of which seem to be a) British and b) thinly disguised soft-core.  They're generally hilarious.   A great example of this is The Vampire Lovers with Peter Cushing, a film based on the short story "Carmilla" by Sheridan LeFanu. 
Onmyoji I & II
Dolemite - If only for the line, "While you were away, I had the girls learn kung-fu."
The Faculty - just about everyone in it went on to become huge.

and most of the movies mentioned already.

BTW, Little Shop of Horrors was a musical on Broadway for years before becoming a movie musical.