Formal:
* Extraordinary
* Remarkable
* Exceptional
* Outstanding
* Phenomenal
* Splendid
* Magnificent
* Impressive
Informal:
* Awesome
* Brilliant
* Fantastic
* Incredible
* Terrific
* Wonderful
* Superb
* Rad (slang)
* Epic (slang)
More specific:
* Astounding (for something shocking or surprising)
* Breathtaking (for something visually beautiful)
* Mind-blowing (for something intellectually stimulating)
* Unbelievable (for something difficult to believe)
The best synonym will depend on the specific situation and the tone you want to convey.