I have to agree with everyone who's said this is a great thread! Brought back lots of memories.
For me, the first great opening act I ever saw was UK with Bill Bruford, John Wetton, Allan Holdsworth and Eddie Jobson at the Santa Monica Civic Auditorium in 1978. They did the entire first album and repeated "In The Dead of Night" for the encore.
The main act was Al Di Meola doing the Casino tour. When he came out for his encore, he said he'd like to introduce "3 very special friends." Chick Corea, Lenny White and Stanley Clarke came out and they did a 45 minute improv of "Romantic Warrior" mashed with "Duel of the Jester and the Tyrant."
No one was "better" than anyone else that night. It was simply 8 of the best musicians on Earth at the top of their game playing for people who loved them. A truly transcendental experience.
The only other one as memorable was seeing a young Joey Bonamassa at all of 10 or so doing a set with his dad's band at a place called the Beginning II in East Syracuse NY. We saw him play a couple of years later with B.B. King in the Landmark Theater in downtown Syracuse. 5 or 6 years later, he was showing up on PBS specials. We can truly say "we saw him when..."