Hmm, am I the first to add Frank Loesser in here? Not only for "Guys and Dolls," but a few years ago I saw "The Most Happy Fella" for the first time. I went in knowing fairly little about it, and was surprised how much I enjoyed it.
If I only get one, though, I think I almost have to go with Richard Rodgers; I have a CD of R&H overtures, and the wealth of beautiful melodies, even from the lesser-known scores, is overwhelming.
"In my own little corner..."