Who were the only 6 star generals?

George Washington and John J. "Blackjack" Pershing, their tiles are "General of the Armies" and both were promoted to the grade posthumously

More info...The titles "General of the Armies" and "Admiral of the Navy" were deemed to have represented a grade higher than five stars, but there has never officially been a six star designation. Five stars is the highest grade in any of the US armed forces.