Personally I think it is Ron and Hermione. I say this because if you see throughtout the series, the two have a this constant arguing over nothing - like an old married couple. Hermione knows that there is something, like before the Yule Ball - yes, I'm a girl but just admit you like me...
Harry even knows - he often shares eye contact with Hermy, after her and Ron have just had an argument. Just not Ron.
Ron is so blatantly jealous of Krum when he finds out about him and Hermione in GoF. And in OoP he is very much interested in the pen-pal letter that Hermy is writing to Krum. If only he can put his feelings in words.
I found the scene between Harry, Ron and Hermione in OoP very interesting. Harry has just spent his summer in hell, and wants Ron and Hermy to explain the OoP to him...they have this great united front. Makes me think of my parents, dealing an angst filled young teenager...not me...my brother.
Then I think its Harry and Ginny. We already know Ginny has had this massive crush on Harry, CoS onwards. But now she has moved on from that and we see more of her personality on OoP. She is this feisty young woman.
She has a connection with Harry - she was possessed and nearly killed by Voldermort and Harry and Voldermort will eventually battle till one of them falls. They can easily chat now and share chocolate in the library...
Plus on JKR's website, she mentions that Ginny is the first female child in generations...I don't think JKR decided on that and just told us about it because it was useless trivial information.
And at the end of OoP, when Ginny says that she dumped Michael...Ron looks up happily at Harry, as if to give his blessings on the relationship.   ;