In terms of film music, I would say he's the most successful of all-time. His success is far-ranging and encapsulates many different genres of film. The only rivals I can think would be perhaps Jerry Goldsmith, Ennio Morricone or John Barry, but even each of these composers come up short in overall career retrospectives compared to Williams. The older Hollywood composers like Korngold, Steiner and Herrmann don't quite measure up in terms of overall impact either, but, of course, they're from an older generation, so perhaps this isn't a fair comparison.
I like John Williams, but I don't listen to any of his work (even though I do own several soundtracks of his music on disc) or any film music for that matter. I'd rather devote my listening time to composers that I love and adore.