For me, it's when he's followed the door salesman to his home and thinks his quest may be over when he sees a pair of brown trousers carrying a door - and just when you think you'll see Nematzedah's (sp?) face, he goes behind the donkey.
The ending, no question, where his friend hands in homework for him, and we know that he did that without being asked, despite a big risk and investment of time, saving his friend's butt from detention and punishment. That a child would do that for his friend - I don't know, that's just a level of complexity which you don't expect children to work on, and yet, Kiarostami makes it believable.
The ending is just perfect. The way the director built up tension on how the teacher was going to react. And then his "very good" was a huge release to the point of provoking tears on the audience. Cut to credits. Unforgettable film.