I think Ran....... Just because they've been friends ever since they were little and they are childhood soul mates. They have a great chemistry together and both care deeply for each other. Though Shinishi is now Conan, he still tries everyway he can to protect Ran. And Ran would wait for Shinishi no matter what. Plus, they both admit (Not to each other directly) that they like each other.
Ai and Conan could be really good friends. Ai's been through a lot in her life. With her parents and sister both dead and a huge organization after her. It's hard for her to make friends and to get too close to people because she never really knows who to trust or not. And now with Conan and the Professure. She might have felt like she found true friends. Sure, she acts really cold on the outside, but she is a decent person on the inside.