Commander in Chief. He was elected to two terms as the U.S. President from 1869-1877.
Union General US Grant was promoted to the rank of Lieutenant General by President Lincoln in March of 1864. Grant was only the second US general to hold this high rank which gave him control of all Union forces. Previously only George Washington held this title.
Sergeant Major of the Army is the highest rank NCO.
Considering that we have had two men who have held the rank of General (that is higher than Colonel) I would say two. They were Grant and Eisenhower.
The rank of lieutenant general was established by the Confederate Congress in 1862. Out of respect for George Washington, who held this high rank, the Union waited until February of 1864 to do this. Basically this rank was requested by US President Lincoln for his new general in chief, US Grant.
General John J. "Black Jack" Pershing was the highest ranking general in the US armed forces. His rank was General of the Armies, and no other US general before or since has held this rank. Pershing was authorized to design his own insignia, but instead he kept his four-star general insignia.
Given that the highest rank possible in the US Military is Commander-in-Chief, and that the Commander-in-Chief is the President, they all held the highest rank possible. THE ABOVE answer I found here is incorrect. The Commander-in-Chief position is civilian, not military. The whole point of making the president C-in-C is to have civilian control of the military, unlike in other countries in the 18th century whose military did not answer to the government. The president is not a member of the military. I am told George Washington was the only six star general in history (before being president, of course), but I have not checked.
James Stewart held the rank of Brigadier General in the US Air Force Reserve. However, it could be argued that Ronald Reagan would be your answer. He didn't achieve the highest rank in the military per se, but he did become Commander In Chief of the Armed Forces when he became President of the United States.
general of the Army
The President is the Commander in Chief. Highest Rank of the Military.
The highest Non-Commissioned Officer rank in the US Army is Sergeant Major of the Army. The current SMOA is Kenneth O. Preston.
The highest rank in the Marines, by title and rank, is the Commandant of the Marine Corps. The position is currently held by 4-Star General James T. Conway. The position holds 4-Star General rank, regardless of the officer's actual rank. For example, a 2 or 3 star General can be selected and confirmed by the Senate for that position, and thereby gains 4-Star rank by virtue of holding that position, but would revert to their former rank when they left. That's why most retire after serving (your pension is based on highest rank achieved), though not all do.
George Washington was the only general to reach the General of the Army rank before U.S. Grant.