A person who's "born to rule" belongs to the ruling class, or aristocracy, and is "noble" just by being in the family line — whether they have done anything noble or not. In an aristocracy, a princess who doesn’t visit sick children or clear land mines is still a princess.

"Rule of the best" or "by a privileged class" are 16th-century Latin meanings for aristocracy. There are examples of the "best" coming from ordinary society, no matter who their parents are, but often the aristocracy is made up of families who start ruling and keep the power in the family generation after generation. Kings and queens, princes and princesses, and dukes and earls, are all titles within an aristocracy, though some really hard-working and noble outsiders may earn a spot in the aristocracy too.

Definitions of aristocracy

n a privileged class holding hereditary titles

Ferdinand and Isabella
joint monarchs of Spain; Ferdinand V and Isabella I
William and Mary
joint monarchs of England; William III and Mary II
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members of the nobility (especially of the French nobility)
baronage, peerage
the peers of a kingdom considered as a group
the collective body of baronets
aristocrats holding the rank of knight
feudal Japanese military aristocracy
Type of:
elite, elite group
a group or class of persons enjoying superior intellectual or social or economic status

n the most powerful members of a society

landed gentry, squirearchy
the gentry who own land (considered as a class)
Type of:
upper class, upper crust
the class occupying the highest position in the social hierarchy