What is the absolute highest rank in the US Army?

Secretary of the Army. General of the Armies of the United States also known as General of the Armies would still technically fall under a civilian Service Secretary administratively due to Title 10 § 3013; GA was only created when GA John Pershing was promoted due to his role commanding the American Expeditionary Forces, a joint Army/Navy/Marine Force, giving him seniority over other Generals/Admirals. The Navy has a similar rank, senior to Fleet Admiral (FADM), known as Admiral of the Navy (AN), held by AN George Dewey. The reason why 5 & 6 star ranks aren't used anymore is due to other countries not currently utilizing anything higher than 4 star rank for active duty personnel; it's been said that using a six star rank would "insult the French" during WW2.

General of the armys

Commander-in-chief, president of the United States

5 star general

General of The Armu

6 Star General. General of the Armies of the US. Held by President, General George Washington.

O-10

Admiral

General of the Army 5 Star General

O-11 general of the army