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

Two categories, those I never saw and those I did See I. Person.

wished I had seen:

Hendrix at Monterey Pop
John Lee Hooker, with Canned Heat
Bonnie & Delaney w/Duane Allman
 Cream, at the Fill ore East and again at the 2005 reunion

Would love to see again

Allman Brothers Band, Syria Mosque, Pugh 1971
Janis Joplin w/Full Tilt Boogie, Frankfurt 1970
Rolling Stones, 1970 w/ Mick Taylor
Beck, Bogart and Appice, 1972

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.