A difficult question, given the various aspects to take into account in terms of evaluating this. Alexander the Great was the most respectable in terms of conquest, though his private life left much to be desired. Cao Cao was a great leader and fairly misunderstood due to biased historical accounts of the Shu-Han, though he only ever managed to gain complete control of Northern China and never took the title of emperor. Julius Caesar had perhaps the biggest impact on human governance and was a great battle tactician, though its more due to the legacy Rome gave him than his actual accomplishments as a leader.
Overall, I'd say either Charlemagne or Constantine. The former's campaigns of conquest were unprecedented when it came to the long-term effects of what his empire meant for European culture, whereas the latter's promotion of Christianity helped make Europe the powerful continent it has been for centuries through the conversion of the pagan civilisations allowing for their more barbaric traditions to be abandoned, the ideas around the nature of God lead to major advancements in the fields of philosophy and art in the Renaissance and the resolve it provided to those practising it during war allowed for Europe to avoid being conquered by several Asian empires such as the Ottomans.