Music in a movie helps tell the story in so many ways but we should remember a soundtrack and a score are not one and the same. Musical scores are usually composed after principal photography is completed. A soundtrack uses music by other artists hand picked by the director. Directors have written screenplays with a song in mind to accompany the scene. Zach Braff did this for "Garden State." Quentin Tarantino originally intended on using "My Sharona" by The Knack during the gimp scene in "Pulp Fiction," but the rights were given to Ben Stiller for "Reality Bites."
The eclecticism of a soundtrack can vary so much. Not just in terms of genre, but also time period. Some of these movie soundtracks use multiple genres, others vary narrow. Some (if not all) are iconic apart from the movie they accompany alone. With the story they accompany, they can help boost the popularity of the songs themselves. The quality of the music alone is part of the criteria, as is how well they help tell the story or set the mood.
This list is soundtracks of movies from the 90s, not movies with soundtracks that are predominantly music from the 90s.