Without doubt the double bill of HELLOWEEN/IRON MAIDEN a few years back in London. Helloween were actually better because Maiden had Blaze Bayley fronting the band and though he was very good he's no Bruce Dickinson.
I'd also have to put Cheap Trick on any of the copious times I've seen them - Robin Zander is one of the best vocalists ever and the band ROCKS. The first time I saw WARRANT also brings back many good memories. cheers. kaos fan.