I would go into detail but English is not my language, I can read it but not write very well.
I would choose Ryoko for the simple fact that Ncrow introduced more nsfw scenes of her than any other character, she has a much closer bond with the main character, she is a character that keeps a balance between being adorable and sexy. As well as having very well animated nsfw scenes.
I forgot to add that the character wears my favorite colors, black and red.
Although I mainly choose Ryoko, I find it impossible not to keep them all.