This is by no means "The Greatest Rock Song of All Time", but it was a remarkable experience that changed my perspective on a certain song, as well as made me LOVE a song I had previously though of as "just, ok". So here's my contribution.
The song in question is "I Was Meant for the Stage" by The Decemberists. I had always listened to this song and thought, "This song has potential, but there's something missing from this recording." A while after that, I had the pleasure of seeing The Decemberists perform at Chastain Park with the Atlanta Symphony Orchestra. After hearing the live version with orchestral accompaniment, I cried. There was no verbal recognition that I could muster. It just overtook me, and I can honestly say the crowd, the orchestra, and the serenity of that evening gave me one of the most memorable concert experiences in recent years.
While I could not find a youtube, or video clip of the performance, I found a website (
http://www.archive.org/details/decemberists2007-07-13.flac16) that has a lossless recording of the entire performance from that evening. Take a listen, close your eyes, and try to be there.