Its a tough choice... so many good bands to pick from... so many concerts... but in terms of which band was the best live in terms of their performance alone... I would have to say either Kasabian or Slash.
I saw Kasabian last year and they were out and out the best single live band I had ever seen. Possibly not the best concert in total (Metallica and Alice in Chains still hold that title[Festivals not included]) But they were absolutly amazing. The show was so good they even released an official live video of their Vlad the Impaler performance.
However, I went to see Slash with Myles Kennedy last week and maybe its just because it is so fresh in my mind but that was probably the best band I have ever seen live. Myles Kennedy had a flu or something (Slash made some comment about it towards the end of the show) so he didnt do much singing, instead he just held the mic out to the crowd, for the most part it was almost like an instrumental show, which was awesome, because it was slash we were all there to see. There was so much energy in the show it was unbelievable. Slash is nearly 50 yet he put on the most intense show I've ever seen. And the best part was I was only ever a few feet away, Infact during the peak of the paradise city solo, he was on his knees right in front of me, literally just out of reach.