The only parts that matter to me are the dialogues and the scenes (i.e. the graphics and ESPECIALLY the fetishes present in the game), the story is only there to justify the presence of the sex scenes.
I'd say "Grace of the Labyrinth Town", an RPG maker with boring dungeon crawler gameplay and a heroine-corruption mechanic that takes a long time to set up, but is very well brought together, with a rich story and dialogue (perhaps too much, given the type of game). But the quality of the scenes...