These are my reasonings for picking what I picked - I'm afraid people won't understand what my goal was in doing this, so here's my insight:
Oklahoma! - I picked OKLAHOMA because it was the first great American musical, and the first hugely successful commercial musical with songs at the top of the charts and on the radio. Plus, it was the first show to ever record an album. It was also one of the first shows that began the formulaic process of putting together musicals - a certain amount of song, a certain amount of love story, a certain amount of ballet, a certain amount of comedy. It started what so many shows tried to accomplish - Carousel, Brigadoon, etc.- and in my opinion, completely succeeded at.
The Music Man - The first truly great musical with a solid, perfect book and score, seamlessly put together. Again, with songs at the top of the charts. This wasn't an easy year for me - GUYS AND DOLLS and WEST SIDE STORY were extremely close seconds, if not a tie.
Hair - The first rock musical. It would forever change the musical possibility that rock and roll music can actually form a brilliant, moving, workable show. So many shows in the future would create a rock score hoping it would follow the success of HAIR, but none have succeeded. Also, the shows' messages and themes, although specific to Vietnam, transcend any time period and essentially remain timeless.
Sweeney Todd - Like Priest say, literally the OKLAHOMA of the later part of the 20th century. Much like HAIR changed the image of musical theatre, SWEENEY took it in the complete other direction stretching it in every imagineable direction with one of the most brilliant scores and amazing themes. Much like OKLAHOMA and SHOWBOAT, it transformed musical theatre and how people looked at it. Many people are saying A CHORUS LINE, but in my opinion, A CHORUS LINE was an amazingly successful show, obviously, but didn't do anything extraordinary to advance musical theatre as an art form.
Les Miserables - Obviously not my favorite musical of the 1980's, but I chose it because it was the first huge, overproduced London import mega success that would set the stage for MISS SAIGON, CATS, PHANTOM, and other later shows of less significance. It set the asthetic standards on Broadway in the 1980's incredibly high - aside from it's brilliant score and moving story. While I love SUNDAY more than anything, I don't feel it did anything to transform the form - certainly not more than LES MIS, or maybe even DREAMGIRLS.
RENT - I don't really need to explain this one, it's quite obvious what RENT has done for musical theatre. Many say RAGTIME - while I won't argue that RAGTIME is wonderful, in my opinion, it was not groundbreaking in any way. THE LION KING would be a close second for me.
HAIRSPRAY - Mamma Mia is a close second - not because I like it (I hate it, in fact,) but for what is has set the stage for in terms of audience-friendly, jukebox musicals. I chose HAIRSPRAY because it is an impeccable example of seamless musical theatre comedy with a new, fresh sounding pop score. I can certainly understand URINETOWN, and certainly agree. I do not agree with, however, THE PRODUCERS or WICKED.
Just my two cents...
"If you are going to do something, do it well. And leave something witchy."
-Charlie Manson