While has many viable leaders for a Military Victory, Charlemagne ranks among the most powerful. He can amass a huge cavalry thanks to his Father of Europe ability, which grants two free Cavalry Units and gives all Cavalry Units +5 Combat Strength each time he enters a Celebration.
Additionally, he can boost Happiness via Military and Science buildings with the goal of having as many Celebrations as possible.
The civilizations that best enhance Charlemagne's ability are Persia in the Antiquity Age, Mongolia in the Exploration Age, and Prussia in the Modern Age.
Charlemagne should pursue Happiness in order to maximize Celebrations and offset any War Weariness accrued while waging wars across the Ages.
Each of these Civilization choices also offers excellent Gold bonuses that Charlemagne can use to either strategically bolster his Army or develop towns/cities in more quickly.