To me (couldn't explain why, just a matter of taste probably) the fundamental thing is having the back of her towards me, or even having her lying on her side on a surface with her legs not pulled apart with me standing behind. (long to describe but at least hope clear

This as opposed to having her chest pointed towards me. Don't know why, but this last setting would turn me off immediately (I may suppose it's due to the sight of vagina somehow).