That's a hard question, I had already been shipping them from the books... But I think the সেকেন্ড movie, the look on Ron's face when he see's Hermione is out of the hospital ward, and the way they awkwardly shake hands... Gold.
I believe it's in the 4th movie when u get the sense how much Ron liked her when he didn't like her to be with Krum.I think Hermione may have caught his thoughts in the fifth movie. For me the it's the 4th one.I think ,even with the potter book edition , the 4th book has put আরো stronger emphasis on their relationship.