It all depends on who's dropping them (Or setting them off). The Russians or Chinese would either be primarily going after military targets or that plus all major population centers, but it would be a ridiculously-difficult challenge to do that without suffering massive retaliation that even if not completely successful could render them so weakened as to be vulnerable to land attack by an unpredictable array of post-war nations with land adjoining borders...and therefore not worth it.
Far more likely, one or a few weapons delivered covertly by a third-world country willing to take a risk on not being successfully identified or just didn't care, probably Muslim. In that case there is no point in going for strategic military targets as there are too many for that mode, and no one to three of them that would decisively cripple our military capabilities. Such an attack would instead be aimed at inflicting the maximum population and political damage, so the short list of targets would be NYC, DC, Boston, LA, SF, Seattle, Miami, etc. (I omit Chicago because it isn't a blue-water port, and I would look for such a weapon to come in by sea shipment; and as far as Detroit goes, what would be the point?).
