Adam Guettel -- his work is only a tiny fraction of my music collection, yet it gets the lion share of my listening time. I think I'll be listening to Light in the Piazza for the next six months straight.
Roughly in order. I'm only listing people with more than one show, or else I would list Damon Intrabartolo, Lopez and Marx, and Hollman and Kotis. Oops, I guess I just did :P