What is the idiom for unique?

There isn't one single idiom that perfectly captures the meaning of "unique." However, depending on the context, you could use several idioms to convey the idea of something being special or one-of-a-kind:

For something truly unique:

* One of a kind: This is a very common and direct way to say something is unique.

* A cut above the rest: This implies the thing is superior and distinct from others.

* A diamond in the rough: This refers to something that is unique and has potential, even if it needs some refinement.

* A rare gem: This suggests the thing is precious and uncommon.

For something that stands out:

* A breath of fresh air: This suggests something is refreshing and different from the ordinary.

* A different breed: This emphasizes the unique qualities of something or someone.

* Off the beaten path: This refers to something that is unusual and not commonly found.

* Out of the ordinary: This simply means something is not typical or expected.

The best idiom to use will depend on the specific context and what you want to emphasize about the unique thing.