General synonyms:
* Unrivaled: Emphasizing lack of competition.
* Unique: Highlighting distinctiveness.
* Unmatchable: Stressing the impossibility of duplication.
* Unequalled: Focusing on superiority.
* Peerless: Similar to unrivaled, but emphasizing a sense of excellence.
* Exceptional: Focusing on the extraordinary nature of the thing.
* Singular: Highlighting the individuality and distinctiveness.
More specific synonyms:
* Unprecedented: For something never seen or done before.
* Unparalleled: For something without equal in quality.
* Unsurpassed: For something that is the best possible.
* Incomparable: For something that cannot be compared to anything else.
Figurative synonyms:
* One of a kind: A casual way to express uniqueness.
* A cut above the rest: Emphasizing superiority.
* A class act: For someone who is exceptional in their performance or behavior.
The best synonym for "inimitable" will depend on the specific context and the effect you want to create.