Why was Charlemagne a great king?

Charlemagne became King of the Franks in 768. During the time he was king he brought nearly all the territories of modern France, Germany, Belgium, Switzerland, and the Netherlands, and much of the Czech Republic, Austria, and Italy under the control of a unified government. It was the most powerful kingdom in Europe. He was crowned Emperor of the West in 800, and he died in 814. Two great nations, France, and the Holy Roman Empire, both traced their beginnings to his empire.

Charlemagne also established higher cultural standards, bringing in teachers to teach the members of nobility to read. His leadership in this respect started a slow but steady growth of literacy and education that lasted into the Renaissance.