In the novelization of the film, the line used was "I have been and always shall be your friend." of Star Trek fame. This Troper had to put the book down for a moment.
After Optimus Prime's resurrection, his intonation to Sam: "Boy, you returned for me." Yeah.
What makes this especially touching is that we've seen him go to incredible lengths to protect Sam, putting himself in danger over and over to keep him safe. And he sounds so grateful, as though the idea of expecting anything in return never even occurred to him.
This Troper's mother always thought that Jetfire's amazement at first Sam saying he'd met a Prime, then later seeing Optimus Prime himself, was heartwarming. The awe he seemed to have for Sam seeing this great person in Jetfire's culture, and the awe he'd later have when he saw him with his own optics. Especially when he'd go on to sacrifice himself to give Optimus a fighting chance at victory.