Definitely eyes... You can tell something (practically everything) when you stare at someone's eyes. You can also tell if they're lying or not. You can tell what they are feeling at the moment when you look right into their eyes. Unless you're clueless, of course...
The eyes, just because you can tell how someone feels or what personality they have (sometimes, personality isn't always that easy to decode) just por looking into their eyes. Especially because when I see a person or creature that makes me sad or scared, it's almost always because of the look in their eyes.
