Formal:
* Magnum opus: This literally translates to "great work" and is often used for a very important or impressive work, especially by an artist or writer.
* Chef-d'œuvre: This French term is similar to magnum opus but is often used for a work of art.
* Tour de force: This means "feat of strength" and is used to describe a work that is extremely impressive because of its skill or difficulty.
Less formal:
* Classic: This implies a work of high quality that is well-respected and enduring.
* Gem: This emphasizes the beauty and brilliance of the work.
* Showstopper: This suggests a work that is very impressive and likely to receive a lot of praise.
* Triumph: This implies a work that is a great achievement and a source of pride.
* Masterwork: This is a less formal synonym for masterpiece.
The best synonym to use depends on the specific context and what you want to emphasize about the work.