Bernard Herrmann is undoubtedly the best. When you compose the amazingly dispate scores for Psycho, Vertigo and Taxi Driver, you're the best in my mind. Marvin Hamlisch comes close, what with his honky-tonk plinky-plonk piano music for The Sting and The Informant! but that's mainly because I'm a sucker for pianos. When it comes to more modern composers, Jon Brion, Cliff Martinez and Carter Burwell never fail to surprise me. Alexandre Desplat is very dependable, but is often capable of real musical majesty (e.g. the Birth score). There's Joe Hisaishi, famed Ghibli composer as well, and countless others that I'm forgetting. But those are the ones that spring to mind.