Formal:
* Uncommon
* Unorthodox
* Exceptional
* Extraordinary
* Singular
* Peculiar
* Aberrant
* Anomalous
* Out of the ordinary
Informal:
* Strange
* Weird
* Odd
* Different
* Unique
* Out there
* One of a kind
The best word to use will depend on the context and the specific meaning you want to convey.