I think maybe the closest Ennis comes to saying "I love you" is "There ain't no reins on this one."
I agree with this one. But, I would add that it's just as important that when Jack asks him how long they'll be together (under Ennis's horrible rule of only 'once in a while') Ennis says "for as long as we can ride it." He's essentially saying he wants to be with Jack for as long as humanly possible. I think this sentence softens the blow for Jack of Ennis's rejection of the cow and calf operation. Otherwise... I think film-Ennis almost exclusively expresses his love physically (we know that book-Ennis is much more vocal about his affection).
Some of my favorite instances of Ennis's love are things like the hand-holding in the 1st tent scene, everything about the 2nd tent scene, his explosion of smiles and kisses for the reunion, the way he caresses Jack's arm in the motel (these caresses sometimes are his only response to Jack's comments... most importantly the "Brokeback got us good" comment... he really rubs Jack's arm there), etc.
This topic is reminding me of all those old threads on the old board about Ennis falling in love first, etc.
As for Jack, well, he just wears his heart on his sleeve for me.