Great question, Lynn! (So lovely meeting you and talking with you, albeit briefly, the other night, by the way.)
I'll take a stab at this: My guess is that Ennis' moment would be when he receives Jack's first post card. The way he holds it so reverently and re-reads it, mouthing the words the second time, and the way we all know he's been aching for Jack all those four years... It struck me the other night that the reason he writes Jack's address from a piece of paper he brought with him to the post office isn't because he's always had his address, but because that post card is so sacred, he's kept it in a safe place where it can't be bent or marred and transcribed the return address on it to a scrap of paper for the post office, rather than bring it with him and risk defacing it in any way. In the short story, Ennis goes out of his way to find a card that depicts Brokeback Mountain to live in the same closet in his trailer with the shirts. In the movie, Jack's first card *is* a picture of Brokeback Mountain.
I guess a close second would be hearing Jack's truck pull up in the parking lot of his apartment, then seeing Jack climb out of the cab. For Ennis, it was always the idea of Jack that most inspired him. But for Jack, it was Ennis himself.