As someone else mentioned, the hit shows are well documented both on recordings and film/video. Where more than one recording are available the rule of thumb is to always start with the original cast from the show's premiere. Though later versions may offer better sound or more complete performances, the original cast members are the ones who spent weeks developing the characters under the supervision of the authors.
Film versions that are faithful are harder to come by and sometimes ruined by Hollywood casting. (CAMELOT, though faithful is a disater!) PAJAMA GAME, DAMN YANKEES, GYPSY and MUSIC MAN (all from Warner Bros) are fairly straight stage-screen transfers.
A good overview of the first century of Broadway musicals can be found in BROADWAY MUSICALS: THE 101 GREATEST SHOWS OF ALL TIME by Ken Bloom and Frank Vlastnik. While some of their choices are questionable (any list has some) - the colour pictures from classic shws are to die for!!!
Harder to track are the flops. Get Ken Madelbaum's book NON SINCE CARRIE for a tour through 50 or so fabulous flops. The ones that left behind cast albums you can seek out.
As for cast albums, to borrow a song from a recent "fabulous flop":
This is the Moment. Over the past 10 years or so just about every musical ever recorded has been reissued on Cd and most can easily be found in stors. Don't wait too long though, some of the more obscure titles will go out-of-print once the serious collectors have bought them up and you will end up having to pay $50-250 to get some of the weirder titles.
DRG has recently brought back several long o/p Capitol shows: NO STRINGS, OF THEE I SING, THE GAY LIFE, PLAIN AND FANCY, and FLAHOOLEY (which, if you haven't heard - get it!)
DRG has also been raiding the Columbia vaults and given us the excellent studio cast of BRIGADOON (even better than the OCR), KEAN, THE HAPPIEST GIRL IN TEH WORLD and OH! CAPTAIN!
SONY has remastered most of the great Columbia Records classic cast albums (MY FAIR LADY, WEST SIDE STORY, CHORUS LINE, ANNIE, COMPANY, A LITTLE NIGHT MUSIC and NINE) but also they have given us beautiful Cd's of HOUSE OF FLOWERS, LI'L ABNER, CANDIDE, BARNUM, BALLROOM, THE APPLE TREE and OVER HERE.
Decca has redone almost all of its cast albums the big hits (OKLAHOMA!, CAROSUEL, ANNIE GET YOUR GUN) and some treasurable flops (SEVENTH HEAVEN, LOOK MA I'M DANCIN', the revival of ON YOUR TOES) and some absolute trash (ANKLES AWEIGH, DESTRY RIDES AGAIN)
RCA Victor did almost all their cast albums before the division was shut down a few years ago. Some of their discs are now deleted (FANNY, HAPPY HUNTING, MILK AND HONEY, WILDCAT, DO RE MI, JENNIE) and these will require a hunt but the big hits have been given deluxe remasters: HELLO DOLLY, OLIVER, HAIR, HOW TO SUCCEED and FIDDLER.
A couple of fun flops to watch out for: SUBWAYS ARE FOR SLEEPING (on Fynsworth Alley) and WHOOP-UP (Polydor.) No one would ever mistake these for classic musicals but both are just so much fun.
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
. Go to the one at Lincoln Center, and you can get anything you want to help you learn about these shows for free. For those who don't, there are pleanty web-sites that are dedicated to sharing information on popular shows. 
