What is the highest military authority in the US?

Practically, the sitting president, as Commander In Chief, has the highest military authority.

However, in 1976, President Gerald Ford posthumously awarded George Washington with the rank of General of The Armies of The United States, stating that he would forever outrank all officers past, present, and future. So technically, he has the highest rank of any American. [Ed note: Washington's rank was a military rank, and, as such, is still outranked by the civilian chain of command]