A few favorites:
The train scene in Risky Business with the track from Tangerine Dream was a very cool mix of image and sound.
James Horner's score to Field of Dreams, most notably "The Cornfield scene" has some very goose-bump inducing moments; his score to Titanic was also pretty amazing work.
Hans Zimmer's score to Gladiator was very beautiful and matched picture well.
Miklos Rosza's score to Ben Hur was a classic for its time, and really set the mood for picture.
Vangelis' tracks for Blade Runner and Chariots of Fire were pretty incredible. The running scene in Chariots of Fire was particularly moving with the score accenting the motion and color onscreen.
To a large degree, it's my opinion that film music owes a huge debt to Richard Wagner, who first made music that was designed for large imaginary vistas and for specific lighting effects in a darkened auditorium. Wagner's operas were, in fact, the first time the house lights were darkened in a theater, and the first time the orchestra was in a pit and not in front of the stage. His work was not theater set to music; it was the opposite, music for visuals. To this day I love Wagner opera, even though the man was a hateful person and my least favorite musical personality. The music was magic.