Michael Jackson, for obvious reasons. Not being facetious - would love to have seen him, and was all set to FINALLY go to those last London gigs...
The Smiths and Zevon are two that I'll always be sad about missing.
Talking Heads are big one I'm pretty sure will never happen either.
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For me it's GIRLS. Been a big fan of Chris Owens and company for a while, and yet they never ever make it to my city on tour
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Lynyrd Skynyrd (original line-up) and Led Zeppelin.
Rage Against the Machine. I was gonna see them around 2000 at the Tibetian Freedom Concert. But Mike D of the Beastie Boys broke his leg and the tour was canceled. The broke up shortly after that. I guess since they've gotten back together they haven't been the same.