Are we distinguishing original score from soundtrack? Scores are performed for the occasion of the film while soundtracks precede the film. Thor is an amazing original score while Shrek is mostly soundrack (although Eddie Murphy's covers are the highlights).
So, I might say The Graduate or Harold and Maude except that much of those were performed for the soundtrack. Topsy Turvy is an all time fave but all of the Gilbert and Sullivan compositions are performed by actors for the movie. Clubfoot Orchestra's original score to Metropolis might be at the top of the list but it was written for the movie years later. Fata Morgana is mostly soundtrack but the Master Musicians of Jajouka perfom the finale. WIngs of Desire is mostly instrumental but Nick Cave gives a performance as himself in the film. Most films with memorable soundtracks still have an original score, even if that score is usually less memorable. If we keep strictly to curration of music that existed prior to the film:
2001 Space Odyssey
Apocalypse Now
The Royal Tenenbaums
The Big Chill
Jackie Brown
Boogie Nights
Pretty in Pink
Saturday Night Fever
Marie Antionette
Until the End of the World