The worst had to be Rick Springfield a few years ago. In Kansas City, we used to have an outdoor festival where they'd generally have several bands play throughout the day with a bigger name act in the evening. Rick Springfield was the headliner. He hardly played any music. He just kept talking with the people in the front row. He'd play some guitar riffs then for no apparent reason, he'd throw his guitar in the air. I watched, horrified, for about 10 minutes then left. It was pretty bad.
The best? Same venue. Moby. I'd never heard of him before & I was pleasantly surprised. Been to 2 more of his concerts...loved both of them.