7. Planet of the Apes
http://www.youtube.com/watch?v=VjcpRHuPjOI Planet of the Apes shares a screenplay credit between Michael Wilson and Rod Serling, he of The Twilight Zone. It contains, of course, one of the most infamous endings in film history, and to read about the writing of the film is to follow the development of this moment.
As detailed in this excellent article, Serling's earliest drafts took place not on the dusty, alien wasteland of the finished film, but a place which, on the surface, looked very similar to small town America. This is one of the few cases on this list where the original version might have been better - the image of ape clothing store mannequins and ape movie theatres with ape movie posters out front is just irresistible. The ending of this first version had the protagonist escaping from the planet, and returning to earth which had also been taken over by apes. Close, but no cigar. Over subsequent drafts, though, the spectre of the Statue of Liberty rose out of the sand and right into film history, where it belonged. Still would have preferred those ape mannequins, though.