I needed an antidote to the thread about what scene most makes you cry. I hope this isn't too simple, and that there's more than one answer.
But for me, the happiest scene is the reunion scene, hands down. Despite Alma's pain. I have learned to ignore that because it is so overshadowed by Ennis' and Jack's happiness. No one was ever happier than they are in this scene, and what makes them happy makes me happy.
Ennis no doubt assumed he would never see Jack again, and yet he clearly hadn't forgotten him, is still pining after four years. Now Jack is here, and Ennis can hardly believe his luck.
Meanwhile Jack has always had more control over their separation; all it took to fix it was his decision to send a postcard. But he can't be sure how Ennis will respond. Of course, "YOU BET" may have been a clue. Still, he had good reason to believe Ennis would be pleased enough to see him but not interested in resurrecting the sexual part of their relationship. And yet here is Ennis, not only going along with it, but actually instigating! It must be unbelievable for Jack, too.
If only the movie would end here!