Well, the highest possible rank in the US Army is "General of the Armies of the United States," a rank considered in the 1950s for retired (but still living) five-star General Douglas MacArthur. Had the idea gone through, MacArthur would've received a sixth star! For various complications, MacArthur declined the promotion, and the proposal was scuttled.
But, in the 1970s, the proposal was revived - not for MacArthur (who died in the 1960s), but for a general who died at the close of the 1700s! You guessed it...
During the American Revolution, Washington was titled "General and Commander in Chief" and held the equivalent rank of a Major General (who wears two stars). When the Quasi-War with France erupted, President John Adams named Washington as the commander of a newly formed American army - to protect the nation in case of a French invasion (which never came). Washington was promoted to the rank of Lieutenant General (a three-star position).
Washington died in 1799, but no one outranked him, until World War I. In that conflict, John J. Pershing was given a fourth star. And in World War II, several leaders were given a fifth star - including Douglas MacArthur, Dwight Eisenhower, and George Marshall.
In the 1970s, Congress appropriately moved to remedy this situation. No one, they reasoned (correctly - in my opinion) should ever outrank America's FIRST general. So, they created the position of "General of the Armies of the United States" - a six star general rank. And they posthumously promoted George Washington to the position.
To this day, George Washington is the only person in US history to ever hold this rank. And he, of course, only holds it in death.
Will we ever have a LIVING six-star general? Probably not. And if we do, will they give George Washington a seventh star? :-)