Without a single doubt, 2001: A Space Odyssey.
I'm a complete atheist, but that movie is the most moving, religious experience of my life. (Absolutely beats to death the mindless hours of droning at the "kingdom hall").
I was lucky to see it when I was 12, and I saw it several other times in my life, and now at 22, I still have yet to see any movie that can communicate so much content with mainly images and very little dialog.
Movies I had to keep secret from my JW parents -- Taxi Driver, Pulp Fiction, A Clockwork Orange, The Matrix (yes, my parents didn't approve of that either).
And the first Godfather is also a shining classic worth a million re-viewings!