Hmm.... thats a tough one, either:
Tool's 10,000 Days Tour @ MEN Arena (Manchester) 2006 - Acoustics were just right, the set was great and the precision and complexity of the performance was amazing. And thats before mentioning the psychedelic visuals throughout!
or
Rammstein's Liebe ist fur alle da Tour @ MEN Arena (Manchester) 2010 - Again acoustics and sound set up was just right, the set was the perfect mix of old and new material and of course the stage show was amazing, naturally featuring giant flamethrowers and of course the infamous Penis Cannon!
really hard to choose between these two as on both occasions the performances where excellent same goes for the visuals, although Tool were technically superior i think i'm going to have to choose Rammstein, solely because they had Combichrist as their supporting act and they were EPIC live.