If you had access to a time machine, what concerts would you go back to see?

Concerts I would revisit.

Percy Sledge free concert in Damrosch Park 1996.

Unknown (to me) guest at the Mozart Cafe getting up from his table and friends and playing the piano while friends were cheering and teasing him. He was not a beginner, that is for sure.

Of course, there are more, but these were out of this world.

Badfinger - 1972 tour for "Straight Up"
Jeff Buckley - anytime & anyplace
Michael Hedges -  anytime & anyplace
Rickie Lee Jones:- 79, backed by Neil Larsen & Buzzy Feiten
Prefab Sprout -  anytime & anyplace
The Who - 1973 tour for "Quadrophenia"
Wishbone Ash -  1972 tour for "Argus"

1972 - Genesis at the Castle, Columbus, OhioStands out among all others in my life. Gabriel in full costume changing for each song and telling stories between each song.Collins with a drum kit from 200 years in the future.Rutherford, seated the entire concert with a rack of 8 guitars behind him, giving a master class on nylon-stringed guitars. One of the most vivid memories of my life.

1973 - Miles Davis at the OSU student commons in the round. He walked among us playing a keytar (don't know if it was called that back then but he was one of the first).

Quadrophenia Tour 1973 was awesome. Almost as good as Elton John and John Lennon at MSG. Early Springsteen at Asbury bars and small theaters compete.
 Would love to have been at Monterey, Bangladesh and Last Waltz (even the movies were outstanding) but did get to see all nights at No Nukes, Dylan 30th anniversary (all good but Bob), Concert for 9/11 and all nights at Rock N Roll Hall of Fame 25th anniversary shows --all at MSG.