Geez, tough one but probably The Eagles back in their hey days if I had to pick just 1. One concert I am really kicking myself for not going to that was not too many years ago was when Alabama was in Ames on their farewell tour. I'm more of a country music fan than most genres so probably a lot of concerts by some of the legends like Cash, Haggard, Jennings, etc. I bet were quite something to see and even Elvis back in his prime was probably quite a deal too.