Gee... Although I may not say it was the BEST concert I ever went to, I would say that the Mars Volta show at last year's Bonnaroo was the most enjoyable concert experience I've ever had. They started a half an hour late (and it was a MIDNIGHT show), but that was the first standing room only concert that I've been to where I genuinely wanted the band to come out and play for another hour or so when they were done. And they played a full show for sure. Usually I'm about ready to leave by that point.
OK, now that I've thrown some variety into this thread...