No the correct word is "better" .The original expression is "Discretion is the better part of valor", it is comparative, not superlative. It is comparing to other parts.But the main reason is that this is an idiom, or fixed expression which cannot be changed. http://idioms.thefreedictionary.com/discretion+is+the+better+part+of+valor It is comparing between being brave or being careful. Therefore we use comparative adjective.

I think is the best