I saw one of Motorhead's first gig's at Birmingham (UK) as a support act to Hawkwind who Lemmy had just left, way back in the 70's. (awesome)
Also a standout for me was David Bowie at Wolverhampton also back in the 70's as he was going through the Ziggy Stardust / Alladin Sane crossover. He played a double set because the support act failed to turn up. Nice of him wasn't it?
And on a trip to the States, I managed to catch Elvis Costello at I think (someone correct me here) an outdoor gig in Marlboro? Virginia.
His support act was The Crash Test Dummies, (ever heard of them?)
It was definitely in Virginia (I wasn't that stoned)
The Smiths before they were famous (I attended a university in Northern England in the early 80's).
They're all good for different reasons, there can't be a definitive best gig for me.
cheers
David