Ran: Some might say she cries a lot and never helps Shinichi and makes stuffs worse and stupid etc. But that's because in her mind she doesn't know Conan is Shinichi, she just tries her best to take good care and protect Conan. Ran always understands Shinichi's thought, when she closed the curtain and said Shinichi won't let anyone die,not scaring that she might get killed. or when she said about justice which she thought Shinichi would have said the same thing and many more. They always share same feeling and same thought, they can even feel the other's in danger even though they are far from each other. Shinichi loves Ran because she's "strong-willed and stubborn, though she cries a lot", crying is also the reason why he loves her and want to protect her. Ran cares about everybody, she even risked her life to save strangers even though she scares a lot. She also saved Haibara twice despite Haibara's cold towards her. She is good at karate, study, household, sing, play piano...Ran always gets attracted from boys but she just waits for Shinichi no matter how long it takes. She's an action girl (but not stupid,she can solved cases by her own one time) and Shinichi is more about mind so they're perfect together.
Haibara: Understands Shinichi if it's about the BO. Haibara does not seem to know Shinichi's personality. She just knows he loves solving cases no and he loves detective. She can also know when he is thinking about Ran because she might has one sided love for him and Shinichi cannot hide his feeling towards Ran very well, but she cannot know what Shinichi would do or would say. Shinichi always talks with her about BO stuff, but not shares with her about his thought of Holmes, of soccer, of his hobby..which he talked to Ran a lot when he is Shinichi. Shinichi considers Haibara as his older sister who need to be protected because she has experience so much pain and he feel responsible for her sister's death,more like sympathy. Besides, she is the only one who has the same situation, who can create the antidote and who has information about BO for him. She is intelligent but she's too "old" for Shinichi. I think he need someone to help him relax after many cases. He smiled a lot when he's with Ran, but he's really serious when he's with Haibara. He also scares of her personality and her mocking, so how can he be happy if he's with her?
I think Ran is more suitable for Shinichi. They love each other no matter what. Haibara deserves happiness but not with Shinichi. She might suits with Akai or Subaru:D
Reallyyyyyyy sorry for the long postttttttt!!
With love <3<3