This thread is another of my not-too-serious attempts at generating some discussion via lists, only this time there is no list, just a singular item. It would be interesting to hear who is considered to be the greatest of all film composers. But also, why is that person the greatest?
I feel that Bernard Herrmann is the single greatest. The reasons are basically three:
1) His music fits the films he scored so flawlessly that in all the films I have seen - which is almost every one he scored - I never once thought "that music is out of place!" or something similar. This is in sharp distinction to some other, even great, film composers. I remember seeing "Woman in White" which had a score by Max Steiner, who is truly one of the greatest, but who in this instance wrote something that was really over-the-top and irritatingly unnecessary. Herrmann never once did that, in all the scores I have heard, and he added so much that the films are absolutely unimaginable without his music. For instance, Vertigo or Psycho. They do not fully exist without the music he created - sometimes against Hitchcock's original directions as to what was to be scored. But even in the lesser films - such as De Palma's "Sisters" or "Obsession" - films that are not perfect by any means - his music was so intensely expressive that it elevated the films far, far beyond what they would have been without it. I am certain that De Palma would agree.
2) He created an art form out of what is most essential to film scoring. Prior to Herrmann, film scores were basically adaptations of opera and operetta music. Herrmann was the first to realize that the Wagnerian leitmotif approach is unnecessary to a film score, and is often in fact artificial. Today, many lesser composers use this fact to create a score out of mere block chords. Whether they know it or not, they are doing something that Herrmann invented, albeit on a vastly superior level. His style uses a direct development of motifs without reference to characters, themes, etc. In other words, a purely musical development of motifs, rather than a leitmotif development/recurrence. It is interesting how recently the leitmotif has had a resurgence in the scores by Howard Shore for Lord of the Rings and of course in John Williams. But these films are long, highly detailed epics somewhat similar to Wagner in the many characters and themes that can benefit from the extra musical "organization" that the leitmotif creates, as opposed to the often mechanistic and artificial approach of Steiner and others who - in the Studio Era - were scoring single 90+ minute features. Though Steiner's work in classics such as King Kong is fabulously great, he was sometimes sabotaged by his unswerving devotion to the leitmotif. Herrmann however bypassed all of this, and created a style that he seemed to gravitate to very naturally, with his straightforward variational/timbral development of extremely simple motival constructions. He was in this regard also the first "minimalist" film composer, though people may not fully recognize that fact due to the extremely espressivo/romantic nature of his music in combination with the films he scored.
3) The last main reason is something that is not really definable, but is simply the musical quality and originality of his ideas. The basic musical ideas of his scores are simply beautiful, whether one is considering the Wagnerian Leibestod-like appoggiaturas of Vertigo, or the "monochrome" avant-garde intensity of the dissonant and rhythmic Psycho, or the elegiac beauty of the viola d'amore of On Dangerous Ground. He had what most composers would kill for, but which cannot be taught no matter how hard you study - the ability to create musical ideas that are simply beautiful. That is why his film scores can be listened to as pure concert music.