In my opinion, I would choose the Buddhist Indian emperor Ashoka the Great (304-232 BC), who was described by H. G. Wells as the greatest ruler the world has seen. In his earlier life, he was a bloody and ruthless conquerer but after conquering most of Southern Asia, he felt regret for all the bloodshed he caused and wanted to repent. After converting to Buddhism, he established peaceful policies that were rare in the ancient world:
http://en.wikipedia.org/wiki/AshokaQUOTE
Policy
Emperor Ashoka built thousands of Stupas and Viharas for Buddhist followers (the Ashokavadana says 84,000 such monuments were built). The Stupas of Sanchi are world famous and the stupa named Sanchi Stupa 1 was built by Emperor Ashoka. During the remaining portion of Ashoka's reign, he pursued an official policy of nonviolence or ahimsa. Even the unnecessary slaughter or mutilation of animals was immediately abolished. Wildlife became protected by the king's law against sport hunting and branding. Limited hunting was permitted for consumption reasons but Ashoka also promoted the concept of vegetarianism. Enormous resthouses were built through the empire to house travellers and pilgrims free of charge. Ashoka also showed mercy to those imprisoned, allowing them outside one day each year. He attempted to raise the professional ambition of the common man by building universities for study and water transit and irrigation systems for trade and agriculture. He treated his subjects as equals regardless of their religion, politics and caste. The weaker kingdoms surrounding his, which could so easily be overthrown, were instead made to be well-respected allies. In all these respects, Ashoka far exceeded even modern-day world leaders.
He is acclaimed for constructing hospitals for animals and people alike, and renovating major roads throughout India. Dharmashoka defined the main principles of dharma (dhamma in Pāli) as nonviolence, tolerance of all sects and opinions, obedience to parents, respect for the Brahmins and other religious teachers and priests, liberal towards friends, humane treatment of servants, and generosity towards all. These principles suggest a general ethic of behavior to which no religious or social group could object.