The single finest acting moment, in my mind, has been mentioned by several already. It's the subtle change and the emotions expressed in Ennis's face when OMT says that bit about the "other fellow, the ranch neighbour". It's so subtle yet it's so clear that these words delivered a devastating and cruel blow to Ennis and he's fighting with everything he has to process this terrible news (and the spite with which they were said) and yet to hold on to some sort of stoic composure.
Best Acting Moment Ever. Though also extremely difficult to watch, for that reason....
Runner-up IMO is the whole reunion scene between Ennis and Jack. I don't know how many times I've watched that, on the net, in the film... I never tire of it. It's my favourite film scene of all time. It shows everything that is between them.. the longing, the love, the passion, caution, joy, confusion, despair, relief, comfort... Dividing it up into still images proves the perfection.. every single image shows a nuanced yet eloquent new set of emotions on their faces, in their body language. (And it's totally hot - let's not forget.
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