This topic is posted 9 years ago, but I just joined this forum recently. I saw this and was interested in it, so it's okay if I only give reply now, right?
Anyway, this is my reply.
In general, Ran is a likeable character. She's kind, caring, trusting, loyal, patient, and skilled in karate. She has flaws like being easily afraid of ghosts and monsters tales, a crybaby, and believing in superstitious things despite being a detective's daughter (fortune-telling for example, which is just plain stupid to me), but those flaws can be ignored easily when she has such positive traits in her. If there's someone like her in real life, I think I don't mind befriending her to some degree. I only need to make sure her negative traits won't transmit to me. So in conclusion, I don't hate her, but that doesn't mean I like her so much either. Indeed she's nice in general, but as a reader, I can't bring myself to be interested in her character. Maybe it's because she lacks character development. Or maybe it's because she doesn't have anything special even though she's the heroine of DC. In any case, you can say she's just "okay" or a tolerable character to me.
But if we talk about who should be with Shinichi, my answer definitely won't be Ran. I can't support ShinRan. In fact, I dislike this pairing a lot. Why?
Because I just don't understand why a girl would date a boy who has been taking advantage of her again and again. Conan has been lying to Ran so many times, mainly for the sake of his identity. Yes, he keeps his identity from her to protect her, but that doesn't mean he can also manipulate her family and friends. He darts Kogoro so many times to solve cases and sometimes he uses Eri and Sonoko. He literally uses them to get closer to the organization and this might endanger their life. Even after he uses them like this, he still keeps the truth from them, endangering them even more. If he told them the truth, at least they will have a chance to prepare for the worst, but if they don't know anything, they can't prepare anything to save themselves when the danger finally comes. Not to mention it might destroy their confidence when finally they figure out the truth (particularly for Kogoro since it will affect his reputation greatly). For now, Ran still doesn't know the truth and since she's kind, loyal, and trusting, she always believes in everything Shinichi/Conan said (see? Shinichi is taking advantage of her good nature), but what if she finally knows the truth? Will she just forgive everything? Will she still want to date him?
A normal, educated girl with healthy heart and mind may forgive, but trust and love are different matters. Why should she date a boy who has been taking advantage of her and her loved ones? Why should she date a boy who doesn't treat her parents well? Her parents are the ones who gave birth to her and raised her. They are people who have loved her even before she was born and will always love her unconditionally. They deserve her love more than anyone else. If this guy can't respect and treat them well, why should she love him? Why should she trust him when he has manipulated her and her family so many times and been putting them in danger without their consent?
If Ran still wants to date Shinichi or even worse, marry him despite knowing what Shinichi has done to her and her family, I probably will have no more respect for her because her decision is very stupid.
What if she never finds out the truth?
I'll blame Shinichi for that. He's so shameless and despicable. He doesn't deserve Ran at all. Honestly speaking, the relationship between Shinichi and Ran has been wrong since the start of the series. This is truly lamentable.
As for Ai, well, I like her. She may look cold on the surface, but actually she's a warm person inside. She's intelligent and dependable. Ran is kind-hearted, caring, patient, and loyal, but so is Ai. Her kindness and loyalty has been portrayed many times when she was with Conan and the Detective Boys. When they're faced with cases, Ai is often babysitting them (taking care of them, watching them so that they won't be exposed to danger) while Conan runs off to catch the culprit and solve the case. Ai always takes care of Professor Agasa's diet because she cares for his health. She always supports Conan even though sometimes she doesn't agree with his idea. She may question Conan's decision, reminding him the danger in his decision, but in the end she always respects his decision and supports him as much as she can. And she respects Ran for who she is. Sometimes she reminds Conan that Ran isn't weak, perhaps to remind him that Ran isn't a damsel in distress who always needs him to rescue her, or perhaps to remind him that he should cherish her more, or perhaps both. And I can say Ai is a patient and strong girl because despite having to grow up and work in the organization, he's still able to keep her kindness, he's still able to keep her sanity. She hates the organization, but she still patiently worked for them to protect her sister, the only family she had.
Of course, Ai has flaws as well. She's hard to trust people, she's paranoid when faced with Black Organization's matters, and she has this sarcastic personality, but it can be ignored and tolerated easily because those flaws are there with a reason. She has those flaws because she grew up in the organization where people almost always harbour malice intention. Those flaws are there to protect her from their malignity. When you're surrounded with dangerous people, of course you can't trust people easily. If you're exposed to danger for a long time, it's only natural for you to be paranoid. This paranoia serves as a reminder for you so that you'll be cautious every time and will notice immediately when the danger is nearby. It can be useful when you're surrounded with danger, but it will become unnecessary, annoying, and troublesome when you're in a peaceful environment. And when you grow up in an environment where there are many people with sarcastic personality such as the members of Black Organization, it will be hard for you not to be sarcastic as well. But at least Ai never uses her sarcastic personality excessively, neither does she ever use it for bad intention.
In conclusion, I like Ai more than Ran because she's likeable as a person and more interesting as a character. And I can accept if Conan ends up with Ai (even though I don't like Conan). They've been working together without having to manipulate each other. Their relationship isn't built on lies, but on trust and respect, and that's what makes their relationship looks more realistic and promising compared to ShinRan's.
So yeah, for me, it's Shiho/Ai for Shinichi/Conan. But it will be better if Ai ends up with someone else who's better than Shinichi/Conan. Shinichi/Conan is a jerk, seriously. He shouldn't treat Ran and her family like that. This is a big and fatal flaw for a main protagonist like him.