Your preference often depends on how you were introduced to the show. If you like the score, it is good to go back and hear how the original creators (coached by the authors) interpreted the roles.
Those who only know CABARET from the (admittedly brilliant) revival will naturally prefer the 1998 cast. Still, you should not write off the 1966 original, since it documents the original performing style. The score also seems less bloated: No "Mein Herr", No "I Don't Care Much", and No "Maybe This Time." (All great songs, but none of them really belong in the stage version of CABARET.) You do gain Jack Gilford's Schultz (and the charming "Meeskite") and the full version of "The Telephone Song" (even if it is done a double the tempo for the recording.)
The SONY reissue CD also includes some fascinating bonus tracks by Kander and Ebb of some cut songs. Not great for listening but good documentation.
For GYPSY, you can never have too many recordings of this score but at least one should be the Ethel Merman original. Though Angela Lansbury and Bernadette Peters weave in some beautiful nuances to their performances, no one brings such ferocious anger to the role, as does Merman. Even if you don't like her voice, it does seem well suited to the role.
There are a few cases where the
original recordings are not satisfactory:
SHOW BOAT - There is no 1927 OCR, but the altered 1946 revival was recorded. This revised version was also given a superb studio cast album in the 1960s with John Raitt and Barbara Cook and it has better sound. The 1993 Toronto cast is perhaps the best single CD of the SHOW BOAT score though again the revival made a number of changes.
CAROUSEL - Decca's cast album is heavily truncated, so the 1965 Lincoln Centre cast with John Raitt is a better choice: more theatrical and with better sound (but the supporting cast isn't as good.) The most complete CAROUSEL on Cd is the 1993 London revival cast.
ST. LOUIS WOMAN - Mercury's CD of the Encores concert cast is much more complete and far more theatrical
ANNIE GET YOUR GUN - where Merman's 1966 revival is far more vividly theatrical than her 1946 original version. (I know a lot of people here prefer the Bernadette Peters version but that does not represent the show as originally conceived and written. True, the 1966 version does eliminate the Tommy/Winnie songs, and adds "Old Fashioned Wedding" but al least these changes were done by the original authors.)
FINIAN'S RAINBOW - RCA's 1960 revival cast has better sound (in stereo!) and is more complete. The Malcolm Gets off-Broadway revival is great but the twin-piano accompaniment pales compared to the full orchestration.
BRIGADOON - the Jack Cassidy/Shirley Jones or Brent Barrett/Rebecca Luker studio versions both give a much better sense of the show
THE KING AND I - Decca's cast album is woefully abridged, and the flat sound is unflattering to Gertrude Lawrence's voice. The 1977 revival with Brynner is much more complete and generally better performed.
ON YOUR TOES - there was no OCR of the original 1936 production, but the 1954 revival got recorded. The 1983 revival, however, is a better recording because it uses the original 1936 orchestrations, is note-complete, and beautifully recorded in digital stereo.
FOLLIES - Tough call here because the original cast was unbeatable but both the RCA concert cast and the Paper Mill cast albums are more complete and give you a better sense of the show.
DREAMGILS - Geffen's original cast disc is pretty useless as a document of the show. The concert cast fills the void even if the cast isn't as good.
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
One of the most interesting Oklahoma!s I've seen was a national tour with the relatively unknown at the time Florence Henderson as Laurey and Barbara Cook as Ado Annie. 
