General Excitement:
* Amazing
* Awesome
* Brilliant
* Fantastic
* Incredible
* Marvelous
* Phenomenal
* Spectacular
* Terrific
* Wonderful
Stronger Expressions of Amazement:
* Astounding
* Breathtaking
* Dazzling
* Extraordinary
* Magnificent
* Remarkable
* Stunning
* Unbelievable
Informal Expressions:
* Cool
* Rad
* Sweet
* Sick (in a positive way)
* Mind-blowing
* Out of this world
More Specific Synonyms:
* For beauty: Gorgeous, stunning, breathtaking
* For skill: Impressive, masterful, brilliant
* For success: Triumphant, groundbreaking, remarkable
* For surprise: Unexpected, shocking, astounding
The best synonym for "wow" depends on the situation and what you are trying to convey.