There are so many really fine Act 1 finales it's difficult to choose. However, the ones that come immediately to mind, for me, are Into the Woods, La Cage Aux Folles, and 1776. They are all powerful, and all for completely different reasons. Dramatically, they life you into a different place. They make you see the world just a little bit differently than you did before. I think that's what makes them successful.
"And the postman sighed as he scratched his head, you really rather thought she ought to be dead..."