This is one time I have to disagree with William (with due dread that he might pulverise his terminal in rage...)
If I say Herrmann for me is a great composer without contextualizing that statement in any way, then what adjective do I use for Puccini? Szymanowcki? Scriabin? Rachmaninov? And then what adjective do I use for Debussy, Ravel, Tchaikovsky? Following that, what adjective would I use for Brahms and Chopin? Then what would remain for Mozart and Bach?
Herrmann was both inspired and characteristic, as well as technically adept. These qualities alone already elevate Herrmann to a select group of composers, regardless of genre, and certainly above most clones of so-called 'serious' music (it's not the objective, it's the results that count).
I have heard most, if not all of the celebrated Herrmann film-music and I always marvel at it. I have only heard one concert work of his, very long ago, and I can't remember it at all. Quite a few people have told me that his concert music is not very impressive, however I have no opinion of my own. If I have to compare Hermann's great soundtracks - which many times can be listened to as pure music, to Prokofiev's output, I would be doing Herrmann a great disservice. I wouldn't know where to begin... Of course there are powerful offerings by Herrmann that I also would rather listen to than some Porkofiev, but that's about it. I won't write an essay today - Prokofiev's achievements and multifarious contributions to the art of music are well documented and easily accessible - but I will bring only one example up: The Hangover Square piano concerto, compared to any of Prokofiev's (or any of his first movements). Herrmann was given the opportunity to compose a concert work without restrictions (artistic or scenic - except the end had to be piano solo), and wrote a work, of which I happily own 2-3 interpretations, as it is good! But it would be like day and night if I have to entertain comparisons with Prokofiev's offerings in the same genre for so many reasons: Depth, breadth, inspiration, piano writing, harmony, rhythm, orhestration, etc. Galaxies apart, and that is not taking anything away from Herrmann who is very deservedly sitting on the high - just not that high - pedestal that his music justifiably afforded him.
That is why 'Perspective' is so important. I recently read on the Internet someone comparing Rolling Stones' contributions to popular music (you know, all genres), as equivalent to those of Justin Bieber. Now tell me again how narrow my field of vision is and about my 'straw' men; tell me how I am buried into the past (although I am young enough), how I refuse to appreciate and equate the quality of the minimal (brain-cell count) school of film-music, when I'm sure someone here - or on YouTube - will start comparing Hans' or Daft Punk's music to Prokofiev's.
Like a theatre composer said, "Would you call someone a novelist if he could neither read nor write?..."