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It is easy to say Ashoka. Lets try to analyse it.
Alexander was a courageous explorer who could not run an empire. Therefore he never can be considered powerful. He had power.
Ashoka had lots of power and he took the most powerful decisions of all times - Peace and no War. The only equal to Ashoka was Chengiz Khan. Bu tthen Chengis was an opportunist unlike Ashoka. Thetrefore Ashoka undoubtedly was the better amongst the world leaders of all times in the Kaliyug.