I'm not sure what you mean by "favorite soundtrack and not score"?
Are you saying, that you are looking more for real music that has been inserted into movies, and not about the music that was actually produced for movies? You then say you want something that gives off the movie's vibe (which IMO is what musical scores do more-so than actual musical soundtracks that were in general, made for musical listening outside of the movie). I'm a little confused, especially since someone already said "The Exorcist".
The greatest soundtrack without debate, at least in terms of innovation is Easy Rider. This is the movie that made putting music into movies popular. This should be in everyone's collection, and I'm very saddened that I have not seen it on this list already by the time of my posting. Incorrect people, incorrect.
Some other favorites would be A Nightmare on Elm Street 4 if you like 80s music, as it's full of 80s pop songs, and even though A Nightmare on Elm Street 3 had "a little" of this (Dokken), Part 4 really boasted the musical soundtrack in terms of musical variety.
An honorable mention would be the soundtrack from Wild Hogs (even though the music doesn't necessarily resemble the movie that much because we've heard the music so many times before the movie even came out). Easy Rider is however, a different story. Most of the music (especially "Born to be Wild") actually premiered during the movie, as with did MANY other songs during the movie that are now popular because of the movie.
Pink Floyd's the Wall is a must-own. Seeing the music in actual context during the movie will change what you think about when you listen to the music.
If you are a Troma fan, I deeply enjoy Poultrygeist: Night of the Chicken Living's soundtrack. The Rocky Horror Picture show soundtrack is also a must own, as well as Tenacious D's Pick of Destiny Soundtrack. Quentin Tarantino has some really awesome classic songs in many of his movies (Kill Bill, Inglorious Basterds, Death Proof) that many people would easily like.
As far as "musical scores" (someone put down the Exorcist which is damn great too), I'd say there's too many good ones to list. Star Wars, Halloween, Big Trouble in Little China, Blade Runner (Vangelis), The Matrix, Batman are just a few, although that's only just cracking the lid on the pot. Blade Runner is absolutely a musical masterpiece, yet alone a visual masterpiece as well. Anything Vangelis, John Williams, Danny Elfman, or Hans Zimmer is going to be worth owning IMO. I would easily consider John Carpenter/Allan Howarth honorable mentions in the realm of horror, easily. Harry Manfredini is more of an "honorable" mention when it comes to musical scores because of his lack of variety. However, his sound is really powerful, and it makes Friday the 13th what it is today.