Formal:
* Excellent
* Superb
* Wonderful
* Splendid
* Remarkable
* Impressive
* Outstanding
* Admirable
* Commendable
* Praiseworthy
Informal:
* Great
* Fantastic
* Terrific
* Awesome
* Brilliant
* Amazing
* Nice
* Cool
* Rad
* Sweet
Specific:
* Effective (for a result)
* Beneficial (for something positive)
* Pleasing (for something aesthetically pleasing)
* Pleasant (for a positive experience)
* Satisfying (for something that fulfills)
* Skillful (for someone's ability)
* Talented (for someone's natural ability)
* Kind (for someone's nature)
* Helpful (for someone's actions)
* Generous (for someone's giving nature)
More nuanced:
* Fine (a neutral positive)
* Decent (a modest positive)
* Acceptable (meeting basic standards)
* Satisfactory (meeting expectations)
* Superior (better than average)
* Exceptional (significantly above average)
The best word to use depends on the specific situation and the level of formality you want.