Usually, a Captain.
Chief of Staff is the title of the highest ranking officer of the US Army. He holds the rank of General, which is an 0-10 / 4 star rank. The title of Commander-In-Chief is given to the President of the US and is the commanding authority for all military forces, but the holder of this rank, while often having served in the military, is not a military officer by virtue of title.
Sergeant Major of the Army is the highest rank NCO.
The current US President .
Anyone in the US Army can lose their rank - in the late 1990s, the Sergeant Major of the Army was demoted. It's not as easy for an SFC to lose their rank as, say, a Specialist or Sergeant, but it can happen.
In the US Army, General.
The highest Non-Commissioned Officer rank in the US Army is Sergeant Major of the Army. The current SMOA is Kenneth O. Preston.
In the US Army, there's two ranks - Specialist, and Corporal. In the US Marines, it's just Corporal. The Specialist rank in the Army is not an NCO rank, whereas Corporal is.
Lieutenant Colonel.
The President is Commander-in-Chief of all US armed forces.
That really depends on their occupation. Captain is normally the rank held by a Company Commander. A Captain could also be a staff officer, there could be doctors who hold the rank of Captain, etc. Some Captains are in command positions; some are not.
General of the Army.
captain