General synonyms:
* Unique
* Characteristic
* Singular
* Special
* Remarkable
* Striking
* Notable
* Peculiar
* Individual
* Uncommon
More specific synonyms:
* For appearance: eye-catching, striking, striking, prominent, arresting
* For sound: sonorous, resonant, mellifluous
* For style: original, innovative, idiosyncratic
* For personality: individualistic, eccentric, quirky
* For features: hallmark, hallmark, signature, defining,
The best synonym to use will depend on the specific meaning you are trying to convey.