I think my genre fan card would get revoked if I didn't mention John Carpenter when speaking of film soundtracks. Or the ever popular "Suspiria" for that matter. For me at least, the aforementioned define film scores. For many people, the music that accompanies a film has a tremendous impact on the whole 'product'. My new favorite product is "The Wicker Man" .
I find that the music within the original 1973 "The Wicker Man" not just adds to the film, it helps create it's structure. The film would not be the same animal without it's very recognizable songs. It tops my list of film soundtracks and scores.
What soundtracks are definitive for you?