Definitely the Act II finale of AUGUST: OC comes to mind. I'll never forget the first time I heard Amy Morton (as Barb) screaming at a shocked Estelle Parsons (as Vi), "I'M RUNNING THINGS NOW!!!!" at the top of her lungs. I think my mouth just opened, no sound came out, I just sat there with my mouth opened during the blackout, then the lights came on and I just stayed on my seat for some time processing what had just happened, then I felt like I just had to have a cigarette.
I love the "Blackout" number in IN THE HEIGHTS, isn't there a blackout right at the ending of "96'000"? I love the lighting of that show.
"Some people can thrive and bloom living life in a living room, that's perfect for some people of one hundred and five. But I at least gotta try, when I think of all the sights that I gotta see, all the places I gotta play, all the things that I gotta be at"