Worst touring act I've seen: Diet of Worms. Yes, with Damond Jiniya on vocals. The tour package was Godhead (my friends' band) first, then Mortiis, then Diet of Worms, then Christian Death headlining. Diet of Worms was so abysmal live that they were driving people out before Christian Death started. ("Punchy" from Christian Death was also part of Diet of Worms.)
It was soo bad that after a few more shows Christian Death "magnanimously" offered to change the tour order with Godhead, having them play a higher slot after Diet of Worms and right before Christian Death. My friends wisely declined, since they actually wanted to play in front of an audience.
(Later I saw Damond Jiniya fronting Savatage and he was terrific...honestly have no idea how Diet of Worms could have been THAT awful, but, there ya go.)
Worst headliner I've seen: Nevermore, performing after Opeth. Warrel's voice was Bad that night and there was a steady stream of Nevermore fans exiting the venue during their performance. This, after half the crowd had already left after Opeth finished. In the words of Jim Sheppard, "Yeah, it's been pretty brutal playing after Opeth on this tour" -- it was Opeth's first-ever US tour -- "but nowhere as bad as Atlanta."
Worst one-off band I've ever seen: Ghost of the Robot, which 'featured' James Marsters (Spike from "Buffy") on vocals backed by basically a high-school band. This was at Dragon*Con.
Lord, he was awful. I was outside the ballroom where he was playing -- since I
do value what little sanity I have left -- hanging out at a friends' band's table*, and people were coming up to us. "Please, make them stop!" "Can't you turn them off or something?!" All we could do was shrug and say "Sorry."
The following band was a band called The Cruxshadows, and a few of their fans had staked out a place on the rail in advance. Having gamely endured the sonic attack** and aural assault that was Ghost of the Talent, the Cruxshadows' frontman gave them free merch to help make up for the audio trauma.
Divine Heresy - They were part of the Firewind, Dark Tranquility, Arch Enemy tour...I am not a violent man but I seriously wanted to take a swing with a bat at the barking vocalist...music sucked the singer continually disrespected the audience. Great way to gain some fans.
I referred to Divine Heresy's lead singer's incessant haranguing as their "mosh-pit beg-a-thon" over on the AtlantaMetal forums....and promptly got that comment sigged.
That was a truly annoying performance.
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* Emerald Rose, for those who know 'em...
** Hawkwind reference. And none of us had our "wheels".....