In today's military, seniority by rank is limited to four-star generals and admirals. And while public law still allows for five-star generals, one hasn't been appointed since Omar Bradley held the rank in 1950.
Yet, six-star general is a rank that (technically) exists.
Two men have held higher ranks in the Armed Forces of the United States. The latest was General John J. "Black Jack" Pershing, whose contributions to service were awarded with the title General of the Armies of the United States, complete with gold four-star insignia. His rank was higher than that of other four star generals due to an act of Congress that mandated that he remain preeminent above all personnel until his death in 1948.
Although I hope the act of Congress didn't specify the year.
Toward the end of World War II, Congress considered promoting Gen. Douglas MacArthur, already a five-star general, to General of the Armies, on the same level as Pershing. The Army Institute of Heraldry even designed an insignia for this rank which included six stars.
But as the years went on and the U.S. came closer to its bicentennial birthday, the idea that someone could outrank George Washington began to bother some in government, including President Gerald Ford. In 1976, Ford would sign a bill which promoted Washington to stand "above all grades of the Army, past or present."
The text of the bill reads:
"Whereas it is considered fitting and proper that no officer of the United States Army should outrank Lieutenant General George Washington on the Army list: Now, therefore, be it Resolved by the Senate and House of Representatives of the United States of America in Congress assembled, That... The President is authorized and requested to appoint George Washington posthumously to the grade of General of the Armies of the United States, such appointment to take effect on July 4, 1976."
News reports at the time referred to his promotion as a six-star general's rank (though there is no mention of the insignia he would wear).