Well, not only is she rich but she's intelligent and a fellow anon.
Let's be honest here, if we had our choice of who to screw once, then it would be a difficult choice. But if going for the total package, Nagi wins hands down. Rich, affable personality if not impulsive, hates school and has the intelligence and resources to ignore it, and loves games, anime and manga.
If you wanted a loli Shana is the only choice, because her body doesn't age. Louise would grow as well but if her sister is any indication the flat chest stays. I suppose if you're into S&M she's a fine choice.