Chicago at the Iowa State Fair. Either 1974 or 1975. Last night of the fair. Good show, but at the end/encore the skies let loose. Being my first concert I really didn't know that encores were normal, so we left at the end of the regular concert and beat getting soaked to the bones.
Best concert may have been Elvis Costello at the DSM Civic Center in support of his Imperial Bedroom album (1983?). Truly amazing and the crowd was into it. He even commented a few days later on his Tomorrow Show with Tom Snyder interview how outstanding the Des Moines audience was.
Second would be Paul McCartney in Chicago 2002 (seen him 8 times, including at the Royal Albert Hall) but this was somehow the best. Tributes to John, then George, then Linda was especially moving.
I was at the same concert! They played 25 or 6 to 4 for the encore and when Peter Cetera sang "flashing lights against the sky" a bolt of lightning cracked right by the grandstand. I stood there with my mouth wide open.