gr33ngirl, you are new here as of today so welcome aboard.
These subjects come up regularly here, usually it is not a list of all-time greatest musicals because you can't really decalre the "best shows of all time" unless you have seen the great, the good, the bad and the ugly! But that's ok you have a wonderful road of discovery ahead.
Start with the Rodgers and Hammerstein classics. THE KING AND I has the best book. CAROUSEL has the best score. Then jump to MY FAIR LADY, WEST SIDE STORY and GYPSY representing the best of the 1950s. The 60s are best represented by FIDDLER ON THE ROOF, MAN OF LA MANCHA and CABARET. The 1970s is the decade of greatest creative growth and also the decade when Stephen Sondheim gained his fame. The Sondheim shows are essential: COMPANY, A LITTLE NIGHT MUSIC and SWEENEY TODD (and the rest). The decade also yielded CHICAGO and CHORUS LINE and just befre the decade ended came along EVITA, still the best of teh Andrew Lloyd Webber shows.
The 1980s were less creative but Sondheim continued with SUNDAY IN THE PARK WITH GEORGE and INTO THE WOODS. LES MIZ was one of the few good shows that achieved deserved popularity. The decade ended on a high note with two back to back hits that were innovative: CITY OF ANGELS and GRAND HOTEL.
The overall decline of Broadwy continued in the 1990s but then there was PASSION (Sondheim again) and RENT and RAGTIME so the decade wasn't a total write off.
So far the first decade of the new millenium has not produced anything truly great but some good shows have come along. THE PRODUCERS, URINETOWN, and AVENUE Q. Still, I find way too much reliance on safe familiar titles (in revivals or new shows based on famous movies or books) and far too many "jukebox" shows built around pop song catalogues. Nothing truly innovative of earth-shattering... yet. But at least the Broadwy theatres are al full and people of your age group and talking about it and that can only be a good thing! (In the 1970s and 80s very few under the age of 30 were even remotely interested in Broadway!)
Do search out some of the cast albums of the older shows above... you will be amazed at what you hear (and regret you weren't around to see these great shows.)
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