Why the aristocrat is best of people? Many suttas (SN 6.11; SN 21.11; MN 53; AN 11.10; DN 27) contain the following phase:


  ‘The aristocrat is best of those people ‘khattiyo seṭṭho janetasmiṃ,
  
  who take clan as the standard. ye gottapaṭisārino;
  
  But one accomplished in knowledge and conduct vijjācaraṇasampanno,
  
  is best of gods and humans.’ so seṭṭho devamānuse’ti.


Why is the aristocrat best of those people who take clan as the standard?
Taken in the context of a conversation between the Buddha and a Brahman in the Ambaṭṭhasutta (DN 3), it is quite clear how this verse is supposed to be interpreted. 

Given the hierarchy of caste, the Buddha argues, brahmans are indeed inferior to aristocrats (i.e. rulers and nobles). He depicts brahmans as subservient to the aristocratic class, and indicates that the aristocratic class is more exclusive -- an aristocrat could become a brahman, but a brahman could never become an aristocrat. 

Thus, this verse should be taken as an argument against brahmanical superiority. Rather -- and this is something repeated in several other places -- a mendicant has worth not on account of their birth, but their sound ethics, great wisdom, and role as a teacher (of dhamma). This is a standard argument against caste hierarchy, which is depicted as illogical given how brahmans are subordinated to the nobility in practice. Instead, the Pali canon places the Buddha and the Buddha alone at the top of a newly configured hierarchy (ostensibly based on ethics and wisdom alone, but sometimes on the basis of Gotama's godlike preeminence among all beings).