I thank you all for playing, but unfortunately nobody got it. It is the brain of course! All other body parts are necessary and various ones can be enjoyed more than others, but the brain adds the aspects of anticipation, memory (an action triggers reminiscing of another time, place, person, etc.), and fantasy.
btw I would have got the right answer on your quiz if I had gotten over here sooner! Damn, I always miss out on the prizes!