Random question to encourage reader participation: what's your favorite instrumental/orchestral theme from a movie (that is, non-vocal music)? Some movie scores don't have one big tune that's recognizable as the "theme," but it's fair to say that many if not most movie composers try to include one big melody that will define the picture. The themes for Star Wars or Kings Row or Superman (wait, they're all basically the same theme) stand for those movies in our heads; they are much more important to the image of the movies than, say, poster logos, which can change many times and be forgotten rather quickly.
I think I would pick David Raksin's theme from The Bad and the Beautiful. A long melody (it really just keeps going and going without a clear resolution, like musical perpetual motion) that's hauntingly beautiful but has that slightly unsettling edge that all Raksin's music has, it's the stand-in for the character of Jonathan Shields and the movie's attitude to Hollywood: tough but ultimately rewarding. Here is the final statement of the theme, from the closing scene -- as usual with Vincente Minnelli, all in one take -- and the closing credits (note that John Houseman, the producer, who'd worked on Citizen Kane, does something similar to Kane by showing clips of all the main actors at the end).