Has to be Dylan at the Fleadh in Finsbury Park a few years back. He followed Van Morrison (who, though morose as ever, was darned good) and was bloody awful. His voice was more nasal than ever I'd heard it, the material was lacklustre and it wasn't helped by a PA mixer with Van Gogh's ear for music.
In the end I gave up on Bob, wandered off and found Moving Hearts playing in a nearby tent. Now that was a cracking set!