Worst sound system--Drive By Truckers in a club type venue. Lots of energy, but could not understand a single word they were singing.
Since the lyrics are a big component of their package, that was kind of disappointing.
Worst effort--B.B. King, circa maybe 1987. smallish venue, Austin, TX. His guitar playing was in good form, for maybe 28 minutes.
Then his bass player, younger understudy, whatever, started trying to urge a standing ovation out of the audience. "Ladies and Gentlemen, Mister BB King. BB King, Ladies and Gentlemen, Mis-ter B. B. King."
Roommate at the time and I still keep in touch, and we still talk about the 28 minute concert. No doubting his talent, but we weren't paying to give him a lifetime achievement award that night. He must have eaten some bad Tex-Mex.