Genghis Khan is the leader of the Mongolians in Civilization I.
Total Threat Level: 6
Musical Theme Inspiration: unnamed original composition
While many rate Gandhi as the most dangerous leader in Civilization I, regular players finding a grey city nearby would rather encounter him than Genghis Khan. As with the real Mongol empire, Genghis is able to expand his nation at a faster rate than any other NPC, and favours military science, which gives him a crucial early advantage.
While other leaders may become technically more advanced towards the end-game, if the Mongols have land to expand into, they will conquer it from the beginning, and annihilate any unprepared opponent.
If there had been memes in the early 90s, this guy would have been the star of them.