My dark horse would be BOS - when I worked there, the Saudis loved flying over to get their medical care at my institution (and a few others in the city). Though to be fair the real rich ones were flying private. Hospitals in Boston (including my former employer) have also entered into partnerships to help develop and consult for new Saudi hospitals not only in Riyadh but elsewhere, so I could see traffic on both sides. However, I agree that all of the other options presented are far more likely!