For shows that either established a trend or became a benchmark I would choose:
1920s - SHOW BOAT - what other show from the 20s is still performed with any regularity?
1930s - PORGY AND BESS - Gerswhin's masterpiece (or is this not considered because it was an opera?)
1940s - OKLAHOMA! - because it was so phenomenally popular. (I have no argument with choosing CAROUSEL but it did not influence musicals of the next 25 years the way OKLHAOMA! did. The success of OKLAHOMA subsequently brought out the best in Irving Berlin (ANNIE GET YOUR GUN), Cole Porter (KISS ME KATE) and influenced Lerner & Loewe (BRIGADOON), Julie Styne (GENTLEMEN PREFER BLONEDES) and Frank Loesser (GUYS AND DOLLS.)
1950s - MY FAIR LADY - the show that finally replaced OKLAHOMA! at the top of the long run list. (WEST SIDE STORY had more influence on director/choreographer shows of the 1960s and 70s)
1960s - CABARET - the seeds were sewn for the concept musicals of the 70s and it has the best score of the decade.
1970s - SWEENEY TODD - What PORGY was for Gershwin, so is SWEENEY Sondheim's masterpiece
1980s - LES MISERABLES - Another "opera" on Broadway. Hmm that makes 3 on this list. Unlike the ALW shows, LES MIZ commanded respect from critics, audiences and even many theatre fans.
1990s - a troubling decade with few really good shows but several strong ones: KISS OF SPIDER WOMAN, PASSION, a stunning revival of SHOW BOAT (almost bringing us full circle) but for me RAGTIME ranks as the finest achievement of the decade. (I think RENT is good but would have been much better if Larson had lived to do some polishing and rewriting.)
2000 - In terms of influence, PRODUCERS saw the return of the light-hearted irreverent musical comedy. It's good. It's fun. But it's not at the level of the shows form the 20th century. A few clever shows like URINETOWN and AVE Q are among the most original and well written shows so far. WICKED is fun but it is no classic and has not really influenced anything else so far.
How much trouble is the musical theatre in? Well here we are 5 years into the decade and no new Sondheim show has opened on Broadway since PASSION in 1994. Kander and Ebb's last Broadway show was the STEEL PIER, a quick flop in 1997. That was also the year of the last Cy Coleman show. More and more theatres are booked with Vegas-styled jukebox revues, & second rate revivals of older shows. Among the "new" musicals of recent seasons are re-treads of old movies (MILLIE, HAIRSPRAY - again both light-hearted fun but not influential and not at the level of any of the shows listed from 1920-2000) This past season with PIAZZA, SCOUNDRELS, SPELLING BEE and SPAMALOT at least gave us 4 good shows that are all big hits with audiences. But where is this decade's SWEENEY TODD or RAGTIME or MY FAIR LADY. I'll wait until 2010 before selecting the best of the current decade.
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