It would have to be the World Hunger Year Concert in Detroit in October (?) of 1977. Harry Chapin, John Denver, Gordon Lightfoot and James Taylor played on a revolving stage, backing each other up. Each sang their most popular songs and also sang together. It was magical. No special effects, no large back-up bands, just music as it was supposed to be.
The tickets were originally very expensive but at the last minute they opened up the nosebleed sections and sold tickets for a short period of time for $5 each. A friend and I dared "certain death" (two girls going to that part of Detroit without male protection) and went to the concert. We lived.
Second favorite would be a tie between my first concert (Peter, Paul and Mary) and a Willie and Waylon concert that was a hoot!