well I think that Ai most likely in love with conan , I mean in some kind of episode which I don't remember Ai was very sick and Ran was some kind of taking care of her (making her some kind of hot food sorry I don't remember what was that food ) whatever Ai pretended she was asleep to avoid thanking her or something and Conan was angry with her , but she thought (you don't get at all , do you ? well that's better I don't want you to know either ) see , what was she talking about , i don't think she had a secret that didn't want Conan/Shinichi to know about Ran or something ?
but for Conan/Shinichi i think he just like Ai but his true love is for sure Ran it's very clear through the series and the movies (movie 4 and i think that movie 14 would include so blah-blah and love stuff ).