Although my favorite area during foreplay is my neck or ears I had to choose dick since I can't deny that that is ground zero for excitement in my book...
I have to say though that, although I'm not suprised that balls is such a popular pick, for me, it would have to be last on the list. I know it's an uncommon sentiment but I don't really like having my balls played with at all. It's at best distracting and at worst painful and uncomfortable.