George Washington

Despite his lack of formal military training, George Washington may be one of history's must underrated commanders.
Building an army virtually from scratch, he defeated the pre-eminent military power of his day. Although, he made mistakes, especially early in the war when he composed over-complicated plans that proved beyond the ability of his army to fulfil, he learned from them. He learned how to utilize the strength of his army and strike where the British were weakest, most notably in his famous surprise attacks on Trenton and Princeton after crossing the Delaware River on Christmas night.
However, Washington's true legacy comes from his actions at the end of the war. His ability to walk away from the battlefield, sheath his sword and willingly relinquish the reigns of power made him truly great.