The number one has to be John Williams. His body of work will span the test of time and his recognizable anthems from Star Wars, Indiana Jones, Close Encounters and Jaws are as instantly recognizable as Beethoven’s 9th or Vivaldi's Four Seasons.
Hans Zimmer has had his share of masterpieces; including the Pirates of the Caribbean films, Batman, Dark Knight and one of my favorites, the score of Gladiator; a true example of how film, set to music, can elicit true emotion from an audience.
Danny Elfman was a member of the punk group Oingo Boingo. He has graduated to scoring film. He works a lot with Tim Burton (Sleepy Hollow) on his often off beat visions, but his main stream work includes the soundtracks to the Spiderman movies, the original Batman and Terminator-Salvation which expands on James Cameron’s driving beats to create an opera like score that compliments the robotic action as well as enhances the humanity of the movie.
Even without watching the movie, I love listening to soundtracks the same way I enjoy the classical music of the past. The beauty of David Arnold’s work for Stargate, the eclectic sounds created by Trent Reznor and Atticus Ross for The Social Network, or James Horner's Avatar music, are all as entertaining and as moving as any work from the classical masters.
I do have a favorite, it is the music created by Randy Edelman for the film Dragonheart. If you can listen to the finale without tearing up you have no soul. The soundtrack has a mix of whimsy, drama and grandeur worthy of any symphony. Next time you watch a movie, pay attention to the music and you will hear the new Beethovens, Bachs and Mozarts.