The most amazing opening act I've seen was post-Jerry was NYE at the Oakland Coliseum: George Clinton and Parliament Funkadelic! Holy Shit! I counted more than 20 band members, hangers-on, and small children on the stage together at one point during their set. And Sir Nose's dancing had all the women screaming! It was wild, and you could tell the folks who were there to see George Clinton and Co. didn't know what to make of Deadheads at all! With Jerry, it was Dr. John.
Two of my favorites that come to mind are Little Feat and Grateful Dead at Oxford Plains, ME Summer 1988 and Yothu Yindi and Grateful Dead at Oakland Col. December 1992. Yothu Yindi was particularly out there in light of my state of mind that evening. By the time the Dead hit the stage I was at least 2000 orbits beyond the planet!