I would say Wash DC. The occupants are criminals and so are a lot of the residents. It would have a major psychological impact cutting the head off of the evil snake. Biggest reason I see it not getting picked is that it would be doing the rest of the US a big favor.
Another option, if you want to maximize carnage and destruction would be a large (e.g. 50 MT) surface blast in a place like Raleigh. The resulting fallout cloud would irradiate much of the eastern seaboard up through NE.
One of the things I read is that an air blast doesn’t generate the fallout (dust and dirt) that a surface blast does. I’ve read that within 48 hours most of the radiation is gone and it’s typically safe within two weeks with an air blast. So, while smaller, a rogue state using something like a suitcase nuke would have a tendency to be dirty.
These people aren't going to be using a megaton range nuke, not for the foreseeable future anyway. But then, they don't have to be megaton yield to destroy cities. We proved that with Nagasaki and Hiroshima nearly 8 decades ago.
What these people want is a nuclear device they can easily slip into the country to a chosen target.
And keep in mind that they DON'T need a long range missile, if the use of a missile is desired. Nor even one that's all that accurate. Park a ship off the coast somewhere in international waters (just over 12 nautical miles from land) and launch from there. Any number of the POS missiles we've seen being lobbed around in the Middle East would qualify as a viable missile. The Scud missiles used in the early '90s had a range of 185 miles. Unless we had a heads up on such a ship, there's NOTHING we have in place that would intercept such a missile between the time it was detected and the time it reached its target scant minutes later.
And what targets would be desirable? Well, obviously high value targets like those we've mentioned would be top of the list of Grade-A Prime targets...but the fact is that ANY successful nuclear attack on the United States would be an absolute win for these people.
And these guys have proven more than two decades ago in 2001 that they CAN make a very successful, well coordinated attack of several targets, simultaneously.
Imagine what we would call a "low yield" nuke being detonated from aboard a small boat parked in the shipping harbors of, say, NYC, Galveston, and Los Angeles while at the same time a small handful of boats launched some low yield nukes at random targets within 200 miles of the coastline. Coastline, mind you, of which we have TENS OF THOUSANDS OF MILES worth. Maybe toss in the odd aircraft or two, as well.
Their goal isn't "defeat"...it's "terror". Terrorism being the use of violence and intimidation, especially against noncombatants, to affect some sort of political change/benefit to the terrorist organizations.
To that end, "success" could be the successful detonation of a nuke in the middle of an unoccupied desert in the United States because of the psychological terror effect it would have.