Kitaab At-Tawheed, Chapter: 44 To Be Called Judge of Judges and the Like
It is authentically reported on the authority of Abu Hurairah (ra) that the Prophet 
said: "Verily, the lowest name to Allah is that of a man who calls himself "King of kings," for there is no king except Allah ."1 Sufyaan added: "Like the title Shaahinshaah." In another narration, it was said: "The man who angers Allah most on the Day of Ressurrection and the vilest..."
Allah's Messenger informs us in this Hadith that the vilest, most contemptible, most wretched human being in the sight of Allah is, the one who calls himself "King of kings" or the like, or is so called by others and is pleased with that, because in so doing, he has elevated himself to the position of Rabb, comparing himself to Allah , the true King of kings. Then He makes clear that there is no king of the universe, except Allah , the Almighty, the All-powerful. And here is a warning to all those who take such names and titles for themselves or attribute them to others, without perhaps, fully understanding the meaning or being aware of the great sin which they commit in so doing.
Benefits Derived From This Hadith
1. The forbiddance of naming oneself King of kings or any other name which carries the same meaning such as Emperor of all mankind, Judge of judges etc.
2. The obligation to abstain from using reprehensible expressions.
Relevance of This Hadith to the Subject of the Chapter
That it proves the forbiddance of taking the name King of kings.
Relevance of This Hadith to the Subject of Tawheed
That the Hadith forbids taking or giving such names as King of kings or Judge of judges because this is Shirk in Rabbship.
Footnotes
1. Narrated by Bukhari.