Years ago, I once posted on the subject of royal saints; I have since learned that there have literally been hundreds of such persons. Perhaps this is not surprising, since the Orthodox churches also engage in the practice of canonization. And Anglican Christians also honor saints, even though the Church of England does not canonize anybody.
Aside from those who have been canonized, there also are a number of royals who have been beatified, declared venerable, or recognized as servants of God.
Somehow, though, the one who seems to stand out among them all is Saint Louis IX, the only French king to have been canonized.