My right ear. I know it's strange but if a woman nibbles, bites, licks, and whispers naughty things into it just right I will literally start shaking. I think I could actually orgasm from it.
As some have mentioned...in my case it is the brain. You turn my mind on and the then rest of me pretty much is a musical instrument waiting to be played. Some women play it better than others. This is one of reasons I like a little tease and some serious seduction going into a session.