Well, Webber's Requiem is pretty second-rate (or even third-rate) as requiems go, considering the requiems of Mozart, Verdi, Brahms, Britten, Berlioz, et al ...
As far as composers primarily known for their work in musical theatre who have ventured into the classical realm, I'll take Gershwin's "Porgy and Bess," "Rhapsody In Blue" or "An American In Paris," Bernstein's "Trouble In Tahiti," "Mass" or "Chichester Psalms," or La Chiusa's "First Lady Suite."
"What a story........ everything but the bloodhounds snappin' at her rear end." -- Birdie
[http://margochanning.broadwayworld.com/]
"The Devil Be Hittin' Me" -- Whitney