For me there have been three...when U2 first came out on stage and began playing their fist song of their concert City of Blinding Lights. I have been a U2 fan for a couple of decades, that was my first chance to ever hear them live, and that is a song that reminds me very strongly of my first meeting with a person who means a great deal to me, so that song carries a lot of great positive associations with it.
The second was hearing Beethoven's Ninth Symphony in concert. That was the first piece of classical music I had ever fallen in love with, and still my favorite piece of music of any genre in all the world. When the first singer began his solo, the entire audience let loose with this great sigh, and I knew that everyone, like me, had been sitting on the edge of their seat, waiting for the vocals to start in one of the most grand melodies ever created by mankind.
The third happened recently, at a concert I actually have pretty mixed feelings about. It was the Vienna Philharmonic and they were playing a tone poem about death by Strauss. The tuba player started getting down his with bad self, and when he played, the very walls and ceiling and floor vibrated and you weren't just hearing the music, you were literally feeling it, down to your bones. That was just really cool...represents what music means to me in a nutshell - not just something that you hear, but something that affects you, all of you - vibrating through your mind, body, soul...
now granted it was about 300 people only...and many friends and family...but lemme tell ya how LOUD that crowd was! 
) her admitting from stage she couldn't remember the lyrics to my request and apologizing to me. 
* 
izzat perfect or WHAT. 