Eight shows that are perfect that changed American musical theatre?
Show Boat
Oklahoma
Fiddler on the Roof
West Side Story
A Chorus Line
Les Miserables
Rent
You really can't choose anything else. Those are IT! You only get 7. And to label some of the above as "perfect" is stretching it, too. And just because a show was "perfect" then doesn't make it "perfect" now (case in point: THAT CHAMPIONSHIP SEASON).
For me personally, the eight musicals that challenged my thoughts on how a musical could be done and changed my perceptions include (in chronological order from my birth, when I discovered the shows) [note: not at all including the 'perfect' factor in these]:
Peter Pan
Cats
West Side Story
Bat Boy
Caroline, or Change
Spring Awakening
Passing Strange
Next to Normal
"The Spectacle has, indeed, an emotional attraction of its own, but, of all the parts, it is the least artistic, and connected least with the art of poetry. For the power of Tragedy, we may be sure, is felt even apart from representation and actors. Besides, the production of spectacular effects depends more on the art of the stage machinist than on that of the poet."
--Aristotle