Best:

Pixies: Leeds Festival in erm about 2005 I think. They were amazing from start to finish. The Killers were on before them (who I purposely missed) but their crowd were all ignorant 15 year olds who fucked off before the Pixies took the stage. They completely blew me away though, they're so intense and simplistic.

Iggy & The Stooges: Same year at Leeds Fesival, missed the first few songs but got there just as they started playing 'I Wanna Be Your Dog'. Every second just got better. He's unmatched as a performer and hasn't lost anything despite all the years and the band were unbelievable.

Sigur Ros: Not so much into them any more, but they put on one hell of a show. The closest thing you could get to seeing Pink Floyd in their Heyday.

Worst:

Tool: don't get me wrong, I am a huge fan and everything but they were boring as hell in concert. There's no comradery between the band and they basically play everything as if you're listening to the album on very loud speakers. The effects/lights were good but not that amazing or anything and to be honest I would have rather spent the £45 on a night getting drunk (which was the alternative). Perhaps it just suffered from Phantom Menace syndrome (over-hyped, too many effects but not enough substance etc.) and I know every band is allowed an off-day but I wasn't impressed.

Green Day: The first concert I ever went to see was Green Day and I have to say they were amazing. I still think that. I was 14 but I still believe they were genuinely good. Problem is I saw them about 4 years later with my then-girlfriend for her birthday and, despite having a new album out, it was the exact same concert. Just add American Idiot and a couple of the other 'new songs' on there and you've got the same thing. Same jokes (or lack of), same 'karaoke' bit where they bring up a guitarist, drummer and bassist from the Audience, same covers, same banter and, once again, for £50 of my (parent's) hard earned cash it just wasn't worth it.
Plus, by that point I hated the bitch I was with and Green Day just reminded me of that fact.