What is the rarest dollar bill?

Could you elaborate on what makes a dollar bill considered rare, and what specific attributes or characteristics might contribute to its rarity? Additionally, is there a universally agreed-upon answer to the question of "What is the rarest dollar bill?", or does it depend on various factors such as condition, serial number, or year of printing? Lastly, if there is a specific dollar bill that is widely recognized as the rarest, could you provide a brief description of its features and what makes it so unique?