As a music student I am liable to have many opinions on this subject - here are some of them.

I have yet to find a director who can use already-written music as well as Louis Malle, from Charlie Parker in Le Souffle au Coeur to Reinhardt and Grappelli in Lacombe Lucien and Schubert's second Moment Musical in Au Revoir Les Enfants.

Bergman's use of Schumann's Piano Quintet in Fanny and Alexander is very effective.

Visconti's Death in Venice is the supreme example of classical music and film coming together in symbiosis.

As for original scores, ET is excellent (John Williams' best; his recent efforts are disappointing - the Harry Potter scores suggest very little musical training whatsoever).

I like Krzysztof Komeda's scores for Polanski's Knife in the Water and The Fearless Vampire Killers very much - very atmospheric.

Aimee Mann's soundtrack for Magnolia is outstanding and, unlike many, I thought the scene where the cast sings along to one of the songs really worked.

The score by Maurice Jaubert for Vigo's Zero de Conduite is wonderful.

Colin Tully's music for Gregory's Girl is stunning. likewise Alan Price's songs in O Lucky Man (used as a Brechtian device, as if anyone is interested).

I could go on for literally ages and ages and ages. My admiration for some people knows no bounds. I will undoubtedly think of more, which will find their way onto this post sooner or later.