Though they are rare today, the most common form of rule in world history has been : (monarchy/dictatorship)

The most common form of rule in world history has been monarchy (from the Greek monos, "one," and arche, "power"). It is an unipersonal, hereditary and lifelong form of government. The Head of State is the king (or queen), the prince or the emperor. Although monarchy was at first absolute (the ruler had absolute or total power), it then evolved into a limited form (the ruler needed the help of the most powerful members of society) and finally into a constitutional form (the ruler is primarily a symbolic figure and the nation is governed by a parliament).

At present, the monarchy is the form of government of a few countries around the world, such as Spain, England, Japan and Denmark, to name a few.    

byzantine empire is the answer i think you are looking for.

hope this : )