Answer by Balaji Viswanathan, history buff:
Genghis Khan could be an obvious answer. He built the world's largest contiguous empire. However, one nonobvious answer you might not know is Emperor . I have also considered other strong contenders such as Queen Victoria, Franklin D. Roosevelt, and Qin Shi Huang of China (who unified the nation).
I don't consider power purely through hard power (the power to murder and intimidate people), but also the soft power (the power to influence and get the best out of people). Ashoka was miles ahead on this metric than any other contender. Even in hard power, he was no pushover.
While Khan's empire of 11 million km2 is twice Ashoka's, most of the Genghis Khan's territory was unmanned grasslands. Ashoka ruled over 44 percent of the world population with close to a half of world GDP of his time. While Genghis Khan hardly had the time to rule any of the territories he captured, Ashoka happily ruled for close to half a century.
Besides leading in economy and number of subjects ruled, Ashoka also took a little-known cult of Buddhism and turned it into world's fourth-biggest faith. Ashoka's actions directly impacted more than three-quarters of the known world of his time—from Japan to Afghanistan. Asia was suddenly connected by this new faith and also pacified.
Unlike Genghis Khan, Ashoka quickly realized the folly of violence and impacted a big deal with his culture. For such a huge conquerer, there is hardly any evidence of complaint in any of the cultures he impacted. Even today, he is celebrated across Asia after 2,300 years, and the pillars of truth he left all over the subcontinent are still intact.
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