I think it's rather obvious he will end up with Ran, but like many I'm an Ai x Conan person. I just think Ai is infinitely better personality wise. Ran hasn't shown much personality beyond being excessively kind and lovestruck. She has had no character development at all.
As for Ayumi... she's a child. That's just wrong on multiple levels. The only way she can ever be considered is if Conan is forced to grow up a second time instead of just becoming Shinichi again. Though if that were to happen, Haibara would win over Conan with ease meaning Ayumi still loses.
