How would you describe something that is defying imitation and is thus matchless?

The term that best describes something that is defying imitation and is thus matchless is "inimitable." This word specifically signifies that something is so unique or distinctive that it cannot be replicated or matched by others. When an item, quality, or person is labeled as inimitable, it conveys a sense of exceptional rarity and superiority that sets it apart from everything else.

In contrast, "invincible" refers to something that cannot be conquered or overcome, typically applied in contexts of strength or power rather than uniqueness. "Infallible" indicates an inability to make mistakes or err, suggesting a reliable or perfect nature but not directly addressing the concept of being unparalleled. Meanwhile, "incredible" describes something astonishing or hard to believe, but it does not necessarily imply that something is unique or unable to be imitated. Therefore, "inimitable" is the precise word that embodies the essence of being matchless and resistant to imitation.