I almost wish Brand New was detached from the audience when I saw them. They were totally into the crowd but played awful and Jesse Lacey was being a real weirdo. Definitely the worst band I've seen live as far as that goes, but luckily I haven't been devastatingly disappointed by anyone that I can recall. I hate to say it, but Rush was even a little disappointing when I saw them 2 years ago. I guess I went in expecting them to be what they were back when Rush in Rio came out. Oh well.
Definitely the best band I ever saw live was this band called Damiera (they're in my top 3). They sounded just like as if someone was playing their record over the PA, and they were jumping around and everything. Amazing.

best : Mushroomhead
Worst : A7X

best: bad manners
worst: metro station

Best .. slipknot
worst... sadly atreyu

Can't believe Deftones and Nine Inch Nails have been pinned as being BAD live. What on earth?! I can only assume the people who said that are fucking idiots because there's no way that either band can be terrible live, I have seen both and both have been utterly fantastic and have never spoken to another person with a different view on that.

Anyway for me, best live band I've seen is quite hard to pinpoint. Pixies would be my choice, naturally, but that's more of a sentimental thing and I imagine if you weren't as into their music as I am, it would probably be quite boring. The Flaming Lips, Nine Inch Nails, Sonic Youth, Arcade Fire, Prince, Bon Iver, Yeah Yeah Yeahs, Stevie Wonder and Dinosaur Jr come very close though.

Worst band I've seen live? Probably The Libertines, Kids In Glass Houses, The View or The Living End. Hard to choose again.

The Hold Steady made me very sleepy when I saw them live.

Enter Shikari has to go under my worst too. I managed to get on the barrier for the second stage at Glastonbury a few years ago so I could have a good spot for Yeah Yeah Yeahs and Brand New and they played right in the middle of both and I literally fell asleep on the barrier during their set. Utter gash.

Best - Primal Scream
Worst - Blind Melon

The best live band I've ever seen was Rodrigo y Gabriela. Forget about your fancy stage shows and light shows, it was just a guy and a girl sitting on a stage weaving sheer magic from their acoustic guitars. It was captivating.

The worst live band I've ever seen was Maximo Park. Not only was their music painfully dull and middle of the road but they could barely even hold it together live. Their frontman was so drunk he didn't even know what song they were meant to be playing and their keyboard player could just about stand up, let alone play a note. They were just a mess.

I went to a festival a couple years ago when Alice in Chains first started touring again. Seeing them was virtually the only reason I purchased tickets and bothered to go the festival. Since Alice was headlining their set was scheduled for like 10:00 pm or something but I wanted to get my money's worth so I went to the festival when the doors opened. At this festival I had the severe displeasure of watching Hollywood Undead perform a live musical abortion for 40 minutes or so. They were so unbelievably bad, not only with their cheesy rap/rock sound but also with their juvenile crowd participation urgings and whatnot. Just awful.