Washington's greatest legacy may be his performance as our first president. He could easily have been far more powerful than he chose to be, but instead he avoided becoming a dictator. Other revolutions have not been so fortunate in their early leaders; one has only to contrast the American Revolution with the French and Russian revolutions to see the enormous difference that Washington made.
Washington's great military achievement of winning the Revolutionary War. Without this victory, the United States of America could never have been formed. And, had the war not be won, all the great leaders of the colonies such as Benjamin Franklin, John Hancock, John Adams, Thomas Paine, Patrick Henry, Thomas Jefferson, etc. would have been executed.
Another monumental accomplishments is that Washington refused to be elected king. Had he not done this, the democracy that America has been would not have come about.
Washington had so many accomplishments, it's difficult to choose just one. I guess if I had to choose, it would be how he held the Revolutionary Army, and therefore the Revolution, together through the winter of 1776-1777, almost by sheer will power alone. There were several times in those six months where the army could have disintegrated but didn't. If at any one time, the outcome of the Revolution came down to one man, it would have been Washington during that winter, and there are very rare instances in history where you can say that about any one person being that important to so momentous an event.
Everything Washington was able to accomplish later as President came about because of his military victories in the Revolution.