FDR promoted several American military officers to the rank of 5 Star General and the equivalent Fleet Admiral rank with the order of their promotion determining who ranked highest. Some you will recognize but Leahy and King won’t be among them.
Fleet Admiral William D. Leahy 15 December 1944
General of the Army George Marshall 16 December 1944
Fleet Admiral Ernest King 17 December 1944
General of the Army Douglas MacArthur 18 December 1944
Fleet Admiral Chester W. Nimitz 19 December 1944
General of the Army Dwight D. Eisenhower 20 December 1944
General of the Army Henry H. Arnold 21 December 1944
Fleet Admiral William Halsey, Jr. 11 December 1945
Leahy and King were number one and number two in the US Navy command. King answered to Leahy. That’s in the Navy.
In the military, all branches— Leahy was the senior rank over all the others on your list.
From 1942 until his retirement in 1949, Leahy was the highest-ranking member of the U.S. military, reporting only to the President. He was the first, de-facto chairman of the joint chiefs of staff (before there was one) and personal pal of FDR. As such, even George Marshall had to go through him to FDR. He didn’t do to well as Ambassador to France in containing the Vichy govt from their love affair with the nazis.