Having been to over 150 gig, I feel I am pretty qualified to answer this(still no expert, still havent seen Muse and loads of others), but seriously, no one beats Iron Maiden. I saw them 2 summers ago on the Somewhere Back in Time tour, otherwise known as the most awesome show ever. And it was so amazing worlds can't describe.
Honorable mentions - At The Gates, Metallica, In Flames, Gojira, Pendulum(saw em on Tuesday), Opeth, Firewind, Dimmu Borgir, Immortal, Korpiklaani(most siprisingly awesome band live ever).
Go see any of them bands and you wont be disappointed.