There are 2 kinds of soundtracks
1 - Score made FOR The film - Star Wars, LOTR, Gladiator, Requiem for a Dream...etc
2 - Compilation - Stuff picked because it suit the movie - American Pie, Forrest Gump
3 - Songs written for the movie, not many of these, Magnolia is one example. Basically an Aimee Mann Album.
The best one (or the most listened one) for me is Magnolia soundtrack, fav is the Forrest Gump one, it's basically a soundtrack 0f the 60's and 70's.