My "Best/Worst" concerts were:
1) Courtney Love showing up at an Evan Dando concert in 2001. He was drunk and argued with the audience the whole show which was impressive in its own right. Then Courtney showed up and made him replay "Into Your Arms" (since she missed it earlier), then took his guitar and played Doll Parts and Jennifer's Body, flashed her breasts, told us how she got high with Oasis, and flipped off the audience.
2) Cat Power w/ Dan Bejar opening, also 2001. Dan and Chan both had major tantrums/breakdowns (seemingly both from equipment malfunctions and bad monitor levels) on stage. I love their music, but I left that night thinking that both could be musically transcendent or they both could join the 27 Club. Glad it worked out for the best.