Formal:
* Unique: This is the most direct translation and the most common option.
* Exceptionnel: This emphasizes the exceptional quality of the item or person.
* Incomparable: This emphasizes the lack of comparison, highlighting the uniqueness.
Informal:
* Pas comme les autres: This literally translates to "not like the others" and conveys the idea of being different and special.
* Un modèle unique: This translates to "a unique model" and emphasizes the individual nature of the item.
Literary:
* Irréparable: This means "irreplaceable" and emphasizes the value and uniqueness of something.
* Inimitable: This means "inimitable" and suggests that something cannot be copied or imitated.
The best choice depends on the specific situation. For example, you might use "unique" to describe a piece of art, "exceptionnel" for a historical event, and "pas comme les autres" for a person with a unique personality.