I have too many favorites, but my fave Heath/Ennis expression - When he plays with that lighter, looks nervously out of the window, smoking, drinking, waiting for Jack, wondering if he'll show up. I think he looks so utterly beautiful and vulnerable in that close-up.
My fave Jake/Jack expression - (there are too many, so I'll pick the first one that comes to mind):
It'd be the close-up of Jack's face when he sees Ennis sulking up the hill. He grabs the rope to go lassoo Ennis, but he stood for a minute, looked at Ennis before he goes up to him. The expression on his face at that moment spoke louder than words, his eyes softened, his lips slightly parted, his head slightly tilted...he was in love! I love that scene.