Can't spork, but any Dead anything from 1977 is a great answer.
I'd also co-sign the above mentions of the Allman Brothers 1970 Fillmore East concerts.
Likewise, Prince is way up on my list...the greatest live performance I ever saw, and there's not a close second.
One artist I've been lucky enough to see a number of times, whom I believe to be underrated as a live performer, is Willie Nelson. I don't think people realize what a great guitarist he is. A troubadour in the truest sense of the word.