Soundtracks:
The Lost Boys
Dazed and Confused
Superfly (Thanks Mr. Mayfield)
Purple Rain (Duh)
Pulp Fiction
O Brother, Where Art Thou?
Hedwig and the Angry Inch
The Rocky Horror Picture Show
Boogie Nights
This is Spinal Tap
Harold & Maude (The artist formerly known as Cat Stevens)
The Graduate (Simon & Garfunkel)
Good Will Hunting (Danny Elfman/Elliott Smith)
Scores:
To Kill a Mockingbird - Elmer Bernstein
Amelie - Yann Tiersen
Star Wars - John Williams
The Fountain - Clint Mansell w/ the Kronos Quartet
The Lord of the Rings - Howard Shore
Fantasia - Various
Good choices! I wasn't really thinking of soundtracks with songs when I saw the OP's question, but I love the soundtracks to O Brother, Where Art Thou?, Harold and Maude, and The Graduate. And of course The Rocky Horror Picture Show and This is Spinal Tap! All these are great movies with great soundtracks!
I like the songs from Purple Rain too, but I haven't seen the movie. I've just heard the songs.
My favorite movie score composers? Two stand out for me: Danny Elfman and John Williams. I also really like the score to The Good, The Bad and The Ugly as well.