Stephen Sondheim
John Kander
Jerry Herman
are all that are left from notable composers. There are no more composers who can match the quality or the out put. Who are there equals? No one in my opinion.
IMHO, Jerry Bock is the greatest living show composer, far superior to all the living greats mentioned so far. It's a shame he's been unable to get a show on Broadway since 1970. Four of his works, She Loves Me, The Apple Tree, Fiorello and Fiddler on the Roof are among the greatest musicals ever. (The idea here is not to knock the recently-departed by comparison, but suffice it to say that CC wouldn't have as many in the top tier.)
But here are a few others: Charles Strouse can weave a pretty good tune. Some of his shows (All American, Charlie and Algernon) may not be well known today, but they've some wonderful songs in 'em.
I think David Shire is a wonderful composer (and he's part of two worlds that we have fans of on this board - film scoring as well as musicals) and achieves a level of sophistication, now and then, unknown to Kander and Herman.
The aforementioned Harvey Schmidt and Alan Menken certainly have done wonderful work.
Coincidental, ain't it, that I had Lost In Boston III already in the CD player? Some great Coleman tunes on that - I'd just recommended Big Fat Heart to someone yesterday, and Pink Taffeta Sample's among my favorites.