Expressing Surprise or Amazement:
* Amazing!
* Incredible!
* Fantastic!
* Brilliant!
* Astonishing!
* Unbelievable!
* Mind-blowing!
* Breathtaking!
* Stunning!
* Outrageous!
Expressing Approval or Admiration:
* Fantastic!
* Excellent!
* Superb!
* Wonderful!
* Terrific!
* Impressive!
* Remarkable!
* Fabulous!
* Splendid!
* Magnificent!
More Casual:
* Whoa!
* Holy cow!
* Sweet!
* Awesome!
* Cool!
More Formal:
* Remarkable!
* Impressive!
* Exceptional!
* Extraordinary!
The best word to use will depend on the specific situation and the level of formality you want to convey.