Oh come on... BEST is easy.... WORST is a lot more fun....
Mine:
Prince and the Revolution opening for the Rolling Stones 1981 in Los Angeles, wearing purple lingerie ("a grape that fell into a vat of pubic hair")... suckOrama
"Black Sabbath" and I put that in quotes... 1985 or so in Los Angeles. If I hadn't been in love with the girl I was with I would left. Spinal Tap was better.
Pretenders at UCLA, 1983ish... now don't get me wrong (lol), Chrissie is one of my ALL TIME FAVORITES.... but this night, she sucked... Bow Wow Wow blew them off the stage.
John Fahey at McCabes, Los Angeles 1983 or so... He was so drunk he could hardly play. Sad.
Elliot Smith at The Breakroom, Seattle 2001 or so... He sat down in front of the crowd and proceding to ignore everyone that wasn't in the first row. He sounded exactly like his records and added nothing to the show... we left midway through.
Elvis Costello at Benaroya Hall, Seattle 2004... OK, for the first hour or two, it was pretty cool... but FOUR encores??? Each one lasting 30 minutes or more... ugh, END IT ALREADY.
I'm sure there were a lot more... but those are the standouts...