Washington is considered the "greates" because he was the first person in recorded history (yes there maybe others but not wih such wide ranging impact) that "Voluntarily" gave up his power at the end of a war and then again after two terms in office as President. It was the act of giving up power which helped to established Democracies which was a continuation of the Age of Enlightenment started in Europe. Without Washingtons action there would be no Democracy, America, or Europe as we know it.
Early in the French revolution one man was held responsible in large measure for saving the republican revolution, Napoleon. Yet in the end that same man betrayed that revolution by becoming an Emperor. When all had been taken from himnow imprisoned on the isle of ElbaNapoleon said: "They wanted me to be another Washington."
When George III heard that Washington was thinking of not running for a third termhe had been unopposed for the first twohe said that if Washington went back to his farm after his public career he would be "the greatest character of the age."