General:
* Adept
* Able
* Competent
* Expert
* Proficient
* Talented
* Gifted
* Masterful
* Dexterous
* Adroit
* Cunning
* Savvy
More specific:
* Artistic: Virtuoso, accomplished, ingenious
* Physical: Agile, nimble, athletic, deft
* Mental: Sharp, astute, ingenious, brilliant
* Technical: Knowledgeable, experienced, practiced
The best synonym will depend on the specific situation and the type of skill being described.