Formal:
* Sole: Emphasizing that something is the only one of its kind.
* Restricted: Implying limited access or availability.
* Private: Indicating something is for a specific group only.
* Select: Highlighting a careful choice or limited selection.
Informal:
* Elite: Suggesting a high status or privilege.
* High-end: Implying a luxury or premium quality.
* VIP: Short for "very important person," emphasizing special treatment.
* Limited edition: Indicating rarity and desirability.
Other possibilities:
* Solitary: Focusing on being alone or separate.
* Unique: Emphasizing being one of a kind.
* Proprietary: Implying ownership or control.
* Reserved: Suggesting something is not for everyone.
Choose the word that best fits the specific meaning you want to convey.