I haven't seen this before, great question!
For me it was a hard choice between "you'll run the sheep of again...."
I think Heath/Ennis's smile at that moment was so sweet and full of real true affection or real love, and
"Anything interesting up there in heaven..."
Which was the one I decided to go with because it was a rare moment of such true peace and feeling between them.
I don't think any other single moment was so pure and relaxed, except for the Dozy Embrace, probably my most favorite moment, but so much sadder in a way.