Power across time can be measured as control of global gross domestic product (GDP). The more control an individual has over global politics and industry.
Emperor Ashoka ruled over 44% of the world’s population and controlled almost half of global GDP. His actions and decisions had an impact on citizens living everywhere from Japan to Afghanistan. Even at the height of the British Empire, it didn't come close to Ashoka's level of economic dominance. The British Empire controlled 21% of global output at its height.
Enjoying swathes of control over GDP in 250BC took much less power than controlling a significant share of GDP today. The ancient economy was so underdeveloped it did not take much power to seize control. A company like Amazon, which accounts for almost half of all US online purchases, has carved out a far greater share of economic power in real dollar terms than Ashoka did.
Power is relative. However, one measurement of power could be an individual's control over global gross domestic product (GDP). In these terms, Ashoka emerges as one of the most powerful individuals of all time.
[P1] Power can be quantified as an individual's control over global GDP. [P2] Emporer Ashoka controlled more of the global output than any other leader in history. [P3] Therefore, Emporer Ashoka is the most powerful leader in history.
Rejecting the premises
[Rejecting P2] Controlling GDP output in the ancient economy required far less power than the ability to control GDP in today's post-industrial economy.