What really is the highest rank?

Yes, we all know the US military is under civilian control through the president as commander-in-chief. But, what is the highest military rank? I’ve heard that during peace-time its a four-star general. But then there’s the 5-star general (aka; general of the army, like Eisenhower) during war-time. What I’m asking is that during some research, I’ve come upon a 6-star ranking. Does this even exist? Or is it another confusion between sources? Looking for answers from former-military.