40's-OKLAHOMA - the first of the genre
50's tie - GUYS AND DOLLS & WEST SIDE STORY - one of the first great true musical comedies and then the first musical to successfully challenge the form as it existed.
60's - GYPSY - in my humble opinion, it took a form that was always somewhat fluid, and perfected it.
70's - A CHORUS LINE - the first major Broadway musical to chronicle backstage life onstage. something genius about being able to pull that off.
80's - LES MISERABLES - Clearly the king of the British imports, defined an era in musical theatre.
90's - RAGTIME - Brought contemporary writing to an art form that was suffering greatly and made it accessible without compromising it's roots.
This Decade (though we're only halfway through it) - I have to say (though this is based on my belief that the musicals which contribute greatest to the form are those that take that form and find a way to continue it and give it new life most effectively without compromising too much)...that being said, I'll have to say WICKED.
there's my 2 cents.
"There is nothing scary about life, if you are not attached to the results." - Conversations With God