The is (are) the facial feature(s) that best express(es) emotion on its own; the is (are) the facial feature(s) that best express(es) emotion as part of the whole face.

I believe the answer is: eyes;mouth
When we feel positive emotion, our eyes tend to be folded upward and when we feel negative emotions it would folded on the opposite direction.
The mouth in this process would only make the small expression displayed by the eyes become much more obvious.

Me I do I know how to