Wiktionary
n. (context slang English) boss of all the bosses, especially in the mafia, cosa nostra etc. Often used by law enforcement, the media and the public in general to describe a Mafia boss who exerts significant influence on how the Mafia should run.
Wikipedia
Capo di tutt'i capi or capo dei capi, often referred to as the Godfather in English, is Italian for "boss of all bosses" or "boss of [the] bosses". It is a phrase used mainly by the media, public and the law enforcement community to indicate a supremely powerful crime boss in the Sicilian or American Mafia who holds great influence over the whole organization.
The title was introduced to the U.S. public by the Kefauver Commission (1950). It has seldom been given to specific bosses because it could create tension between different factions (otherwise known as families) within the Mafia. Typically the title is awarded de facto to the boss of the most powerful Mafia family.
Usage examples of "capo di tutti capi".
The capo di tutti capi's rings flashed as he arranged the money in his hands.
I may get away with wrecking a president but not a capo di tutti capi!
I guess 'emperor' would be a fancy name like capo di tutti capi, but it really don't embrace all that Rockecenter really controls.