I’d venture not Dallas, as it is the lowest-density city in the country, and a lot less likely to be totally shut down by such an attack. LA on the other hand, although just as spread-out, has a much higher population, and therefore a much higher disruption potential.
LA was safe until a nuke got by Jack Bauer last season. I suggest Al Quida would take out a big soft target like Bentonville Arkansas with Walmart’s HQ and the true economic center of the US.