A Live One is one of the great official releases. I assume we're talking about those, rather than shows downloaded from tapers or ETree.
People are right to recommend Dave and Tim at Radio City as an amazing recording, but I actually found the setlist to be kind of weak. Music-wise I still prefer the original release from Luther College.
Call me crazy but I still get a kick out of the Metallica concert with an orchestra.
But my all time favorite is the Grateful Dead live at the Fillmore box set from 1969. Something about the bluesier PigPen era just gets me. Always has.