George Washington has to rank up there. He definately had the most lasting positive impact on history of all of the major commanders. Compare Washington to his French counterpart, Napoleon. Washington took a poor frontier nation and set it on track to become the greatest nation in the world. Napoleon took the world's most powerful nation (then France), and set it on track to be the third-rate power it is today.
He was a great leader, a great president, and a great man. I don't know if I would rank him in the top few generals. He did not have great tactical skill, or much military training, and he lost more battles than he won. Many times, he and his revolution survived more due to his luck than his skill.
Don't get me wrong. As I said in another thread, had there been no George Washington, there would be no USA. He held the army and the country together by the sheer force of his will and his character. Military prowess, even though that was his first claim to fame, was the least of his abilities.