Formal:
* Vous êtes très intelligent(e). (You are very intelligent.)
* Vous êtes très brillant(e). (You are very brilliant.)
* Vous êtes très doué(e). (You are very gifted.)
Informal:
* T'es trop malin(e) ! (You're so clever!)
* T'es vraiment intelligent(e) ! (You are really intelligent!)
* Tu es super doué(e) ! (You are super gifted!)
More specific:
* Tu as une tête bien faite ! (You have a good head on your shoulders!)
* Tu es plein(e) d'idées ! (You're full of ideas!)
Remember to use the appropriate gender agreement for "intelligent," "brilliant," "doué," and "malin."