What is the rarest Sonic?

Could you please clarify what you mean by "the rarest Sonic"? Are you referring to a rare character or item from the Sonic the Hedgehog franchise, perhaps a limited edition collectible or a particularly difficult-to-find in-game item? Or is there a specific context or context in which "the rarest Sonic" is being discussed? Without more information, it's difficult to give an accurate and detailed answer to your question.