What is the rarest Chihuahua?

Could you elaborate on the concept of the "rarest Chihuahua"? Is it referring to a specific breed of Chihuahua that is particularly uncommon or difficult to find? Or is it a Chihuahua with unique physical traits that set it apart from others of its kind? If it's the latter, what specific characteristics might make such a Chihuahua considered rare? And how is the rarity of a Chihuahua determined, is it based on the frequency of its appearance or some other factor? Furthermore, are there any well-known examples of rare Chihuahuas that have garnered attention in the dog world?