I think Shinichi will be better off with Ran, 'cause she's the one that he loved for so long. Also Haibara still hides a lot of things from Conan,, if she truly loves him she should trust him no matter one. I don't get the "protection" thing at all, like even if it's to protect him, I still don't get it.
I feel sorry for Ayumi though, she has a huge crush on a person 10 years older than her in reality