Now that you've posted the dumbest movie of all time, what would be your idea of the greatest movie of all time? Mine is probably The Red Violin. You know, I've never seen that. It's on Amazon for 10 bucks, so maybe I'll order it, or at least see if it's on NetFlix.
My all-time favorite? Hmmm....you'll laugh - I think most people know I'm a Mystery Science Theater fan. They riffed a bad 70's movie called "Angels Revenge". I know every line and have watched it 100 times - but I still pop it in the DVD player when I need a laugh. The jokes are priceless.
As for my favorite "standard" movie - toss-up between "It's a Wonderful Life", "Casablanca", or The GodFather