The Old Red Hills of Home from PARADE is perfect. I still don't understand how a song could make me sympathize with the Jim Crow south, but it does.
Similarly, The New World Opening from SONGS FOR A NEW WORLD is so well written and stands alone as just a fantastic piece of music.
Four Jews... from FALSETTOS is a pretty brilliant opening.
The Prologues from WEST SIDE STORY and RAGTIME.
Tradition from FIDDLER.
"I shall stay until the wind changes."