Well, its obvious that a million people are going to say Rocky Horror Picture Show. So I won't say it here.
although i did.
Anyway, a movie that I absolutely adore is Kentucky Fried Movie. Its mainly a bunch of different segments, but the main focus is on the Wizard of Oz meets Enter the Dragon spoof.
Monty Python and the Holy Grail. I used to be able to recite the entire movie from any point. It was sad... Now that I own the DVD, I barely watch it... I should though