What is the rarest quarter in USA?

Can you tell me, what is considered to be the rarest quarter in the United States? I'm curious to know if it's a specific year, design, or perhaps a mint mark that makes it so unique and hard to find. I'm interested in understanding the rarity and the potential value of this particular coin. Could you also elaborate on what factors contribute to the rarity of a quarter and why some quarters are more sought after than others?