Interesting story from the Chicago Tribune,
George Washington Voted Britain's Greatest Military Enemy
Although the article wrongly credits the victory at Saratoga to Washington (that goes to Benedict Arnold), everything else is on the nose. Washington may have lost most of the battles he fought in, but the few victories he had severely hurt the British war effort. He was very good of at protecting his army from serious defeat and attacking when the enemy was most vulnerable. You need only to look at the Yorktown Campaign to appreciate this.