General Synonyms:
* Remarkable
* Exceptional
* Outstanding
* Uncommon
* Unusual
* Impressive
* Noteworthy
* Remarkable
* Exceptional
* Phenomenal
* Extravagant
More Specific Synonyms:
* Astonishing (for something that is surprising)
* Brilliant (for something that is intelligent or talented)
* Magnificent (for something that is impressive in size or beauty)
* Prodigious (for something that is impressive in size or quantity)
* Unprecedented (for something that has never happened before)
Formal Synonyms:
* Extraordinary
* Extravagant
* Distinguished
* Eminent
* Illustrious
The best synonym will depend on the context of your writing.