1638–1715 · King of France, the Sun King, reigned 72 years
Louis XIV reigned for seventy-two years, built Versailles partly to keep the nobility where he could see them, and centralised French government to a degree no monarch before him had managed. If you needed one sentence to define absolute monarchy, he is the obvious mouth to put it in — which is exactly the problem. He wrote a great deal himself, including memoirs of instruction for his son, and the political theory in them is careful and dynastic rather than swaggering: the king is the servant of the state’s permanence, not its replacement.