Formal:
* Virtuoso: Especially used for musicians and performers.
* Proficient: Skilled and competent.
* Masterful: Having or showing great skill.
* Expert: Having, showing, or involving a high level of skill or knowledge.
* Adroit: Clever and skillful.
* Dexterous: Skillful in using the hands or body.
* Accomplished: Highly skilled in a particular area.
Informal:
* Ace: Excellent, especially at a particular activity.
* Whizz: Someone who is very skilled or knowledgeable.
* Guru: An expert or leader in a particular field.
* Wizard: Someone who is very skilled or talented.
* Crack: Extremely good at something.
* Pro: A professional, someone who is very skilled.
The best word to use will depend on the context.