Composer? For me, Richard Rodgers. Not only for his musicals but also for his concert pieces: Carousel Waltz, Slaughter on 10th Avenue, Victory at Sea. His gift of melody is matched by a sense of musical adventure. Exhibit A: the score of CAROUSEL.
I would say much the same about Gershwin.
Stephen Sondheim writes brilliant scores but with Sondheim the text leads the music (not a bad thing) and as beautiful as a show like PASSION is it would be less effective without the text.
Cast albums are NOT "soundtracks."
Live theatre does not use a "soundtrack." If it did, it wouldn't be live theatre!
I host a weekly one-hour radio program featuring cast album selections as well as songs by cabaret, jazz and theatre artists. The program, FRONT ROW CENTRE is heard Sundays 9 to 10 am and also Saturdays from 8 to 9 am (eastern times) on www.proudfm.com