Dic: chief, principal, and their synonyms

Synonyms: chief, principal, main, leading(1), foremost, primary, prime

These adjectives refer to what is first in rank or in importance.

• Chief applies to a person of the highest authority: a chief magistrate.
  Used figuratively, chief implies maximum importance or value: her chief joy.
  
  
• Principal applies to someone or something of the first order in power or significance: their principal source of entertainment.
  
  
• Main applies to what exceeds others in extent, size, or importance: the main building on the campus.
  
  
• Leading suggests personal magnetism, a record of achievement, or capacity for influencing others: one of the leading physicians of the city.
  
  
• Foremost emphasizes the sense of having forged ahead of others: the foremost research scientist of the day.
  
  
• Primary stresses first in the sense of origin, sequence, or development: primary school. It can also mean first in the sense of "fundamental": the primary function of this machine.
  
  
• Prime applies to what is first in comparison with others and to what is of the best quality: a theory of prime significance; a prime Burgundy.

.... The American Heritage