Riyad Us-Saliheen (Gardens of the Righteous)

Chapter 320
Prohibition of Addressing somebody as 'The King of Kings'

1724. Abu Hurairah (May Allah be pleased with him) said: The Prophet (PBUH) said, "The most disgraceful man near Allah is a man who calls himself (or likes others to call him) Malikul-Amlak (i.e., king of kings).''
[Al-Bukhari and Muslim].

Commentary:  Allah appreciates humility and humbleness and abhors pride and haughtiness. To arrogate to oneself the title of `The king of kings' reflects pride and not humility and is, therefore, forbidden. Moreover, this is an Exclusive Attribute of Allah and it is unjust to associate it with any one else.