I feel very strongly that this category must be split into two categories, soundtracks of existing recordings and soundtracks composed for the movie. I think the first is relatively simple to do (see both "Dazed and Confused" and "Almost Famous"). It’s like a songwriter who samples and in my mind is cheating because they can’t come up with original melodies on their own. The second is much more difficult and must be done after the film is shot to supply the correct mood to what’s going on in the movie.
I think a movie like Being There with Also Sprach Zarathustra, Beethoven’s Symphony N°6 in F Major (opus 68), Basketball Jones Featuring Tyrone Shoelaces, Shubert’s Symphony No. 8, I’d Like To Get To Know You, and It’s a Beautiful Day in the Neighborhood is a good choice for the first type and Star Wars: Episode IV: A New Hope is a great choice for the second type.