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

World Series of Rock at Cleveland Municipal Stadium featuring Journey, Foreigner, Electric Light Orchestra and one other band that I can't remember.  Amazing afternoon and night!

1971 Santana concert at the Allentown Speedway in PA.  Not normally a great venue, the audience is sitting in aluminum bleacher seats with the band on the other side of the racetrack, concession stands behind and underneath the bleachers.  Concert starts, all going great, when I smell smoke; turned around and the hot dog stand behind the bleachers is shooting 6' flames at the power lines!  Looked cool for about 30 seconds with huge flames in the dark night sky and Santana playing, but... ZAP.  All the lights went out, including, of course, the band's amps.  We assumed the concert was over and people started getting up to leave, but after about 1 minute we heard... conga drums.  Awesome drumming for almost an hour until the electricity came back on, and the band finished out one of the finest performances I've ever heard.  Definitely time-machine worthy!

Hendrix and Jefferson Airplane at Monterey Pop
Credence at Woodstock 
Any Zep before 1973
UFO with Michael Shenker 1978 Chicago
Ramones Rainbow Theater 1977
Rory Gallagher Montreux 1979
Ozzy with Randy Rhoads
Stevie Ray at the El Macombo 1983
Any thrash band in the Bay Area 1983

@Eonsx311, I attended the 5/23/94 Pulse show in Atlanta, which was indeed great. It is one of the far too few DVD's that I have that came from the same tour or the exact show I attended.....but, I also saw Pink Floyd on 6 - 23 - 73 in Detroit on the "DSOTM" tour. If I had to chose one, I'd probably have to go with the 73 show, because I remember far less of that show.

@laxiotakis, I agree that if I could only pick one music festival to go back in time to see, it would have to be the Monterrey Pop Festival. But, the Atlanta Pop Festival (aka Byron Pop Festival) would be a close second. Either way, you would have seen Jimi Hendrix. I have multiple friends who lived in Georgia at that time who could not go because they were too young to drive, and their parents wouldn't let them go.