it's interesting, prior to showboat, different 'movements' all had a role in the formative aspect of the American Musical. Blake and Sissell, Kern and the Princess Shows, Vaudeville, The Black Crook, Alexander Cohen, the Follies, the George White Scandals. In addition to dancers/choreographers like Ballanchine and DeMille and...and...and...
the idea of the musical is that it could have only been produced by America in the way that it was because of the influx of immigrants into New York at the time and their living in such close proximity in and around New York. The Melting Pot as historians love to call it, is really an impetus for all this artistic/entertainment development.
But i don't think it's hyperbolic to say that with Showboat the synthesis is established and with Oklahoma! complete.
My list:
Showboat
Pal Joey - Ist real anit-hero
Oklahoma! - The seamless vision of Hammerstein II and Rodgers that really legitimizes the new musical comedy form.
West Side Story - Use of dance to tell large parts of the story
Hair - influence of younger artists taking the reins from the older generation
Company - concept musical - but really it's not that nothing happens, it's the development of the themes - which one would argue is significantly different than plot. what it's about versus what happens. Company really explodes onto the scene with this idea fully utilized and integrated
A Chorus Line - I've never really embraced this show as a concept musical. started the public, and the workshop process. but also reached parts of america with a reviatlized sense of The American Theater. it's working class themes resonated as we were spriling into recession.
Evita - JCS was i think somewhat of an anomaly. But with Evita, a SUPER HIT, it became a form that was - from a box office standpoint legitimized. With Cats, that moment is completed and we have a goodly number of European imports within that mold.
Sunday in the Park with George and Assassins - both shows seem to push the limiits of what a show can be about and how to tell the story; kaleidoscopic, meandering, overtly intellectual or political. Mainstream - and this is an important parameter - mainstream shows are rarely so adventurous.
Floyd Collins and Caronline or Change - both are what i would consider community character studies. each has a central character study at play but each also brings the same level of seriousness to most if not all the supporting characters in such an integrated way.
Adding Machine - a truly harrowing experience. and a musical. it is now that the anit hero and the political commentary and the 'non-hummable' musica and the fractured structure really seem to be elements of a production that seeks to reflect how we live.
if the theater and art in general has a central role, i would argue that it's to reflect society's positions - morals - ethics - the way we live with one another. it's not life, it's a refraction - a way of understanding how we live together.
Namaste