Damn tough question. The idea that your choice isn't the Beatles kind of disturbs me. Here I thought I knew something about the world and you go and give it a series of high kicks. By a leprechaun in a raspberry leotard none-the-less. One of my biggest musical regrets is not seeing Nirvana play live. I was of concert going age when they were together and touring too. That's the part that stings. The fact that I could have seen them. I wouldn't choose them though. Without over thinking it, I would probably say Hendrix playing the star-spangled banner at Woodstock. Iconic. To be in a sea of people for that one brief moment...
I thought about the Beatles but don't have a specific show or song. It would definitely be in the Hamburg days, pre Beatlemania, back when they were playing 12 hours a day. That would be the shit. But I can't think of a particular show or song. -kind of cheating. I saw a lot of bands in the 90's and Nirvana wasn't one of them. I always assumed I'd see them eventually. -definitely a bummer.