The one that first came to my mind was "We'll Take a Glass Together" from Grand Hotel. The first time I saw the show was before the Tonys, so I had no idea what was coming. Nor, I suspect, did the rest of the audience.
Michael Jeter started dancing, and it was like a shot of adrenalin hit the entire theater. Everyone sat up and leaned in closer. And the number just built and built from there.
It was absolutely thrilling. A moment that can only happen in live theater.
When the show was over, I thought to myself that I was going to second act the show just to see that number again.
I quickly realized that was impossible. (the show had no intermission, and thus no second act)
So I bought a ticket and went to see it again. (and again)