Well, the answer is there were two "General of the Armies," George Washington and John J. Pershing. Pershing had the distinction first in 1919, but in 1976 Congress authorized the President to posthumously appoint George Washington General of the Armies of the United States and specified that he would rank first among all officers, of the Army, past or present.
These titles put Washington and Pershing above all five-star generals. (see
http://www.historyguy.com/5-star-military.htm for the list of those). However, neither Pershing or Washington ever wore six stars on their uniforms. To the best of my knowledge, Washington never wore any stars, and the most Pershing ever wore was four.
sources:
http://www.mdw.army.mil/fs-p10.htm
http://www.history.navy.mil/trivia/triv4-5m.htm
http://hallbiographies.com/professio...emics/31.shtml
Cheers! M2