Who was the American commander of the Atlantic fleet in World War 2?

It is Nimitz
Fleet Admiral Ernest King was the Chief of Naval Operation.
At the time of Pearl Harbor Harold Stark was the head of the Navy - Chief of Naval Operations, and Husband Kimmel was commander of the US Pacific Fleet. Within a few weeks of Pearl Harbor Stark was replaced by Ernest King as CNO, and Chester Nimitz took over in the Pacific. Stark went to Europe and commanded the Atlantic Fleet, and Kimmel was unfairly made a scapegoat for Pearl Harbor. King and Nimitz remained in those jobs throughout the war.
Nimitz
Admirals Nimitz and Halsey were the over all Naval commanders of the American Fleet in World War 2.