fwiw, I agree with many of the common suggestions, especially Angels in America, Our Town, Streetcar, and the Crucible.
I also want to throw out a dark horse: The Kentucky Cycle.
Won the Pulitzer, was a critical rave in Seattle, LA, and DC; bombed here in Gotham -- i'd say inexplicably, but i suspect it's lack of success on Broadway actually can be explained.
Brilliant drama chronicling key themes in American mythology across almost three centuries.
Though, technically it's 9 plays. (Whatever -- Angels is 2!)
"You, sir, are a moron." (PlayItAgain)