What is the rarest in All Star Tower Defense?

Could you elaborate on what you mean by "the rarest" in the context of All Star Tower Defense? Are you referring to the rarest tower, character, upgrade, or perhaps a specific item or event? If we're talking about towers, it's possible that the rarest ones are those that require a significant amount of in-game currency or resources to unlock, or perhaps they're only available during limited-time events. Alternatively, if we're discussing characters, it could be that the rarest ones have unique abilities or are extremely difficult to acquire. Clarifying your question would help me provide a more accurate answer.