For national and international bands? Hate to say it, but, Katatonia. They've been great the past few times I've seen them but the first US tour was terrible. Bad sound, no stage chemistry, awkward, uncoordinated, Jonas was straining for every syllable. Glad they got their act together. The second worst has to be Cradle Of Filth, again with a combo of bad sound and substandard performance. I hear they're doing a lot better these days too. I won't judge any band too harshly over one or two bad shows, or even a bad tour, there's just so much that can go wrong.
I've been underwhelmed by a lot of big-name acts, but I think that's less to do with their performances than how the material translated or whether I even liked them to begin with. Nile played fine the two times I saw them, but I've never been a fan.