I think the most important thing about Tomoe for me is that she has really good judgement. She likes to tease, but she knows when to stop and knows when she's crossed the line. With a lot of the other girls it feels like they are either Ryo's "superior" / senior (Alice, Keiko, Hana, Akane) or his dependant / pet (Ryoko, Shika, Charlotte, Fumiko, Mayu, maybe Yui). Tomoe feels like an equal. And she also is neither needy or clingy nor emotionally distant. I don't know, there's something about her that makes her feel like soulmate material.
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.