If we're purely talking about the live act and performance, than for me it's easily Beck and the Roots. Radiohead is a great band, I've attended two great live shows of them, but they don't stand out because of their live show. They just do an adequate job of bringing the magic from the album to the stage. Which is commendable in itself. I recently saw MGMT and they are crap live. As much as I love them on their album. U2 are great, but they are also super-professional, which means that some of the early magic is gone. My two uncles did see them in the early 80s (their first gigs on the continent were in Belgium) and said they were amazing. They created an almost sacred atmosphere. But no band can keep that alive for close to three decades, especially not if you become "the biggest band in the world" along the way.