What is the idiom which used to describe masterpiece?

There isn't one single idiom that perfectly describes a masterpiece. However, there are many that come close depending on the specific nuance you want to convey:

For overall excellence:

* A tour de force: This means a display of great skill or artistry.

* A magnum opus: This literally translates to "great work" and refers to a person's most important work.

* A work of genius: This emphasizes the exceptional talent and creativity involved.

For artistic impact:

* A masterpiece: This is the most straightforward and common way to describe a great work of art.

* A thing of beauty: This highlights the aesthetic appeal of the work.

* A timeless classic: This suggests the work will remain relevant and admired for many years to come.

For influential impact:

* A groundbreaking work: This emphasizes the innovative and influential nature of the work.

* A landmark achievement: This suggests the work set a new standard or made a significant contribution to its field.

For emotional impact:

* A heart-wrenching masterpiece: This suggests the work evokes strong emotions.

* A masterpiece of storytelling: This emphasizes the power of the work to engage and move the audience.

The best idiom to use will depend on the specific context and the particular qualities of the work you are describing.