The famous king of the Franks who was crowned Emperor of the Romans is Charlemagne. He reigned from 768 to 814 AD and played a crucial role in uniting much of Western Europe during the early Middle Ages. Charlemagne was crowned Emperor by Pope Leo III on Christmas Day in 800 AD, which symbolized the revival of the Western Roman Empire and the close relationship between the Frankish monarchy and the Church. His reign is marked by significant military conquests, cultural and intellectual revival, and administrative reforms, laying the foundation for the Holy Roman Empire and influencing European history for centuries.