What's the rarest thing in Adopt Me?

Could you tell me, in your opinion as an expert in the realm of virtual economies and cryptocurrency, what is considered the most elusive and valuable item in the popular online game Adopt Me? Are there any specific criteria or factors that make this item stand out from the rest, and how does it compare to other rare finds in the game? Furthermore, have there been any notable transactions or instances where this rare item has been exchanged for significant real-world value, highlighting its scarcity and desirability?