Of the classic shows, RCA's 1977 KING AND I (with Brynner) is far better than the tinny-sounding 1951 Decca set.
The 1992 revival cast of GUYS AND DOLLS has better sound and is more complete than the 1951 original.
Merman's 1966 revival cast of ANNIE GET YOUR GUN is far more theatrical than Decca's truncated 1946 cast album. The same can be said for John Raitt's 1965 CAROUSEL vs the 1945 OBC set. (For completeness I prefer the 1993 London cast which has a nearly-complete version of the ballet as well.)
Victor's 1960 album of FINIAN'S RAINBOW has better sound and is more complete than the 1947 Columbia album.
Matthew Broderick's H2$ is overall better and more complete than the 1961 original even though it does cut one minor number.
SHOW BOAT - the 1993 Toronto cast is the best and most theatrical cast album available. (EMI's massive 3-CD set of the original 1927 version is a must, but it is more for archivists.)
The 1983 revival cast of ON YOUR TOES tops the 1954 revival cast. (No OCR was made of the original 1936 production.)
NO NO NANETTE - the 1971 revival is really the only full recording, but the 1925 London cast is available with poor singers and an ancient gramophone sound.
And while not specifically a revival cast, the studio cast recording of BRIGADOON with Shirley Jones and Jack Cassidy is far more theatrical than Victor's original cast disc.
Now on the other hand the following revival cast albums are to be avoided (unless you favour one of the stars):
Tyne Daly in GYPSY (she was in poor voice when recorded)
FLOWER DRUM SONG - 2003 cast is dreadful
CABIN IN THE SKY - 1964 revival is terrible.
COMPANY -both the 1995 Broadway and 1996 London revivals are totally unlistenable (and unnecessary with Sony's fine re-mastering of the original available.)
SWEET CHARITY - the 1986 Debbie Allen cast has been reissued (WHY? It's a terrible recording!) and while some may like Christina Applegate, the 2005 recording is nothing compared to the Gwen Verdon original.
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