I'd have to choose Risa. Sure, everyone thinks she's annoying because of her shallow love for Dark, but don't we all used to have crushes, too? We can all tell that that was mere infatuation, but that's pretty much the first stage we go through in order to experience true love. Even though Risa can be childish and somewhat blunt, she's still a cheerful and friendly girl. Even after she rejected Daisuke, she still considers him an intimate. She also cares about other people's happiness, and knowing that Dark loved Rika in the anime, she decided to drop out of the competition in winning his heart because she knows she can never have him. And she also showed Satoshi a hearty and warm smile as she waved him goodbye in vol. 3. And that was around the time where a subtle intimacy has started to take place between the both of them. Maybe it was because of her friendliness that caused him to have some feelings for her. I mean, why would he treat Risa differently from the other girls who've had a crush on him?
Well, I also like Riku with her personality and good judgement, but my favorite is still Risa. :)