Let's see. In addition to those mentioned above, I'd have to include "I can't stand this no more, Jack."
First (though not necessarily in this order), because I love it whenever one of them says the other's name. Second, because it is so damn sad. Third, because of how this single moment echoes both the alley scene and the "if you can't fix it" line. Fourth, because of the tragic expression on Ennis' face and the way he clutches onto Jack. Fifth, because it is Ennis' way of saying "I love you."
And rtprod, I also love "Me? I don't know," though mostly for his tiny self-conscious smile -- verbally, I always wish he would have elaborated a bit.
I will gladly engage in an Ennis/Heath love fest any time!