Great ones listed so far, and I agree that rose's Turn has to be the motherlode.
Though my personal favorite--it's not quite a breakdown, but a very very emotional moment/reaction that I think counts: I am What I Am from La Cage.
My personal favorite turning point song--Rumble from West Side Story. What amazing music.
Wouldn't Everything's Coming Up Roses work too--the turning point between Rose focusing on Louise instead of June?
Adding a couple more:
I'm Here-The Color Purple
Valjean's Soliloquy-Les Miz
Not Me-Aida (Radames talks about becoming a new person)
I Know The Truth-Aida
The Reckoning-DRS
Confrontation-J&H
Judas' Death-Jesus Christ Superstar
Astonishing-Little Women
Down Once More-Phantom of the Opera
Close Every Door-Joseph...
I've Grown Accustomed To Her Face-My Fair Lady
Back to Before-Ragtime
I Should Tell You-Rent
Your Eyes-Rent
"If there was a Mount Rushmore for Broadway scores, "West Side Story" would be front and center. It snaps, it crackles it pops! It surges with a roar, its energy and sheer life undiminished by the years" - NYPost reviewer Elisabeth Vincentelli