Formal:
* Specialist
* Authority
* Professional
* Connoisseur
* Guru
* Master
* Adept
* Proficient
* Skilled
* Knowledgeable
* Erudite
* Scholar
Informal:
* Ace
* Wizard
* Pro
* Guru
* Whizz
* Hotshot
* Mastermind
* Brain
The best word to use will depend on the specific context and the level of formality you want to convey.