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

The one that got away. In 1975 we had tickets to see Queen at Kleinhans Music Hall in Buffalo dead center 4 rows back. The concert was cancelled last minute and back then you could exchange or return tickets at point of sale. We exchanged them for Jeff Beck and John Mclaughlin at the Century Theater in Buffalo. Bad news is that I never saw Queen and personally I never cared much for their stuff after that time frame but seeing 2 guitar legends on the same night more than made up for it!

Obviously I would see the classical creators/masters...Bach especially....too many to do justice by mentioning.
Miles Davis in the 50's; Charlie Parker and the great bands he was in; Jimmy Reed for sentimental reasons; Jimi Hendrix; Cream in 1968; The Doors just because; peak Creedence.

I saw Pink Floyd DSOTM in Jacksonville, FL about 2 months after the album dropped....they all appeared/performed the whole thing, in Quad...Clare Torry's wordless vocals were mesmerizing. Then, a greatest hits playlist that left not one thing to be desired (save Sid).

For $7.50 I saw the '72 Stones Exile tour in Tuscaloosa, AL starting with Martha Reeves & the Vandellas, then Stevie Wonder (about 6 months prior to Talking Book); then the Stones, with their entire  studio band featuring M. Taylor's impeccable guitar....a R&R high unlike any other before or since. The Stones were like experiencing a black hole and a super nova simultaneously! Thoughts: if the Stones had cancelled after Stevie Wonder's set I wouldn't have cared (at that point) because being in SW's presence was enough....greatness was writ large on him; when I was able to peel my eyes off Mick for a moment I saw Bianca behind the stage in her signature white dress/floppy hat; Keith drove/guided the band...Mick Taylor supplied the flaming icing on the cake.

There were other great ones.....but there's my top two.

Zeppelin
Jimi
Doors
Woodstock
Cream
AC/DC with Bon Scott
Who with Keith Moon

I have seen so many prob near a thousand 
so many i would love to see again...

i would love to go back to Knebworth 1990 just so that this time i might remember not to forget my bag with concert t shirts etc in a locker at the train station

i still wonder to this day if that actually sent the bomb squad in to open the locker that i left behind..

Stephen Bruton with the Resentments on any old Sunday evening @ Saxon Pub in Austin.