Like some of the other guys have said, I'm a fan of the Greek positions that allow me to get a good visual of the action. For me, Greek is not the most physically pleasurable necessarily it's more about the eroticism and more taboo nature of it that is a turn on.
My favorite is probably me kneeling in missionary with her legs splayed wide giving me great visuals of me not only going in but also the way it looks on the out stroke as the friction grips and pulls a little of her along with me for a moment.
Also that position allows me to watch the looks on her face whatever those might be and also let's me see her play with herself or better yet allows me to play with her and perhaps insert a thumb.