I'm more of a Jerry Goldsmith fan, but Zimmer has done some great work. Gladiator comes to mind
Greatest movie music ever? No. No contest. WAYYYY too repetitive between scores for that.
I really like Hans Zimmer, but as far as popular composers go I think John Williams gets the edge easily. I mean come on: Jaws, Jurassic Park, the Star Wars films, the Indiana Jones films, etc. Those are each among the most best and most memorable scores in movie history.
Bernard Herman also gets some consideration for his work with Hitchcock, especially his simply terrifying screeching few notes in Psycho. As far as individual film scores (removing musical and animated films from contention), I think I'd give the nod to Clint Mansell for Requiem for a Dream. Anytime I hear that string quartet I immediately slip into an entirely different world.
To the poster who pointed out the legendary Bernard Herrmann, I couldn't agree more. No one could set a theme better than he, and Hitchcock owes a huge debt of gratitude for Herrmann's unforgettable scores to some of Hitch's greatest films (not to mention Scorcese's Taxi Driver).
Reading the comments so far reminds me that my background of hearing music in film and TV comes with a long appreciation of found or specially manufactured sounds. Hans Zimmer is the best recent example of that with his work on
The Dark Knight's Joker theme, but I was actually even more fascinated by
Man of Steel in that he'd made the connection from the "infinity" of America to steel instruments, including steel sculptures that can be played, with amazing sounds produced for the film. Maybe his music can sound repetitive, or just because of his famous back catalogue, recognisable. John Williams is also extremely repetitive for the same reasons, but what marks Zimmer out is that extra layer of creativity and his attempts to renew himself, as he had to do in particular for each superhero franchise that came his way.
I class Bernard Herrmann as second only to John Barry as the best ever film score composer. His Hitchcock films and
Taxi Driver have been mentioned but you've also got to take into account his science fiction and fantasy music –
Jason and the Argonauts being my favourite. Incidentally, Herrmann also dabbled with alternative sounds with the theremin on
The Day the Earth Stood Still.
Jerry Goldsmith is excellent but his one weakness I think was always that there was an indelibly American flavour to his music, even in something unrelated like
The 13th Warrior. My favourite of his pieces is
The Mountain from the beginning of
Star Trek V but
Alien's a great score. Once again, sourced, unusual sounds crop up with his use of a conch shell for the alien, and it's interesting that Marc Streitenfeld returned to that creative thinking for
Prometheus, having some of the music written to be played backwards, establishing a strange tone.