"You are the most intelligent one in the class."
Translation:Tú eres la más inteligente de la clase.
Inteligente is one of the adjectives that doesn't change for gender (unlike caro or cara for expensive), so either el or la is correct depending on the subject you are applying the adjective to, in the case of this question there is no context, therefore either should be accepted.
When you're using a superlative - the most or least something, so oldest, prettiest, least attractive, whatever - the construct is to use de there. You're the most intelligent of the class, the prettiest of the girls, oldest of the children, etc, would be a closer direct translation there.