Black Sabbath with Ian Gillan. Probably the worst sound quality possible and absolutely deafening. They played at a large gym which obviously didn't make it any easier for the guy mixing it.

Mountain opening for Purple in '85 comes in as a close second. Leslie West was completely wasted and could barely walk - not to mention play.

And then there's Buddy Guy whom I've seen a number of times now. His concerts are usually pretty cool (he's a phenomenal guitar player), it's just that he likes to insult and curse out his audience - espeically if it's a non-English speaking crowd. ("Don't you f***ers understand English?!?" seems to be his favorite phrase.) Two years ago it had gotten so bad, that I actually walked out half way through the set. Haven't been to one of his shows since.