Guns N Roses in about 93' was the best live band I have seen. Axl had one of his good nights, I heard that the people who went the following night got one of his "I couldn't give a fuck" nights.
Therapy? were brilliant when I saw them in 93' and pants when I saw them about a year later.
I've seen Oasis 3 times, and the first two they were piss poor. The third time they were good, but not to the level that all the hype says they should be.
The Prodigy are always awesome live, at least they were the 4 times I saw them about ten years ago. Manic St Preachers are also pretty impressive.
Green Day and Bon Jovi both surprised me with how good they are live in a stadium setting.
The Chemical Bros are the most pointess band I have seen live, two guys standing behind a desk. Total waste of money.
But the worse band I have ever seen probably goes to Kasabiabn. The singer, I can't remember his name, was off his face and all over the place, kept missing his ques and he was totally out of tune. The rest of the band were obviously embarrased by him, and I was surprised they didn't split up not long after that.