Another good one. And I'm with you -- every expression Heath ever makes is my favorite expression. Especially the ones with a lot of emotion, of any kind: lying on Jack's chest in TS2, reading Jack's postcard, seeing Jack drive up for the reunion, sending up a prayer of thanks, saying "why don't you then?" finding the shirts, looking out the window when talking to Alma Jr. ....
OK, stop me before I just recite the whole movie.
Among expressions that aren't particularly notable, I'm partial to Ennis' profile when Jack is stroking his cheek, and he kind of breathes out and repositions his mouth.
Wtih Jack, for some reason, I like the kind of shrugging face he makes when he says, "course, he hates my guts." I'm betting Jack fans will have some they like better than that one, though.