I can tell you one NOT to get: the 1956 movie soundtrack with Bing Crosby, Mitzi Gaynor and Donald O'Connor (on Decca).
The movie dropped the plot kept only 5 songs, adding "It's Delovely" from RED HOT AND BLUE and 3 new (and forgettable) songs by Van Heusen and Cahn. The performances are not bad but the sound quality on this recording is very poor and many performances are not even from the actual film.
You can also skip the Mary Martin album (on DRG), originally a 10-inch LP covering 6 songs plus and Overture and Finale in very untheatrical arrangements.
The original 1934 version has only been recorded once by a studio cast (on EMI) in 1989. The cast headed by Kim Criswell, Cris Groenendaal, Jack Gilford and Frederica Von Stade is uneven, but the orchestra and chorus are smashing and ONLY on this disc do you get songs like "Where are the Men?", "What a Joy t be Young" and "Kate the Great." The songs are presented in the order heard on opening night in 1934. The show is not performed this way any longer, but this disc gives a good idea of what it would have been like if cast albums were made back then.
Of the revised versions I prefer the London cast with Elaine Paige (First Night) over Patti LuPone's somewhat sloppily sung Lincoln Center theatre version (RCA.) The 1962 cast headed by Eileen Rogers and Hal Lindon (Epic) is lively but the skimpy orchstrations make it less than enjoyable. However this revision includes other Porter songs not used in 1934 or 1987, and this is the version most often staged by community theatre groups.
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