I'd probably try to get evolution in film history, and sprinkling in genres throughout. Start with early silents, like The Cabinet of Dr. Caligari and Nosferatu. Move on to the monster movies of the 1930s, and show the Bride of Frankenstein and The Mummy. Then more golden age of cinema: Hitchcock and Psycho, as well as Peeping Tom. 70s flicks, like The Exorcist and Young Frankenstein (comedy, but I don't care). 80s slasher (not my cup of tea, but I guess it would be essential). Move on to more modern stuff, like Let the Right One In (which I think was an FBG movie club selection).