I don't recall it ever being said. I think most people just call it his favorite because of the way Lois calls Clark "farmboy" and loves to order him around, much like Buttercup in the movie does with Wesley. Also, there's the nice foreshadowing that they'll be together if one considers "farmboy" as a deliberate reference. I think it's possible the writers were big fans of the movie. I'm not that big on the movie, too popular for my taste and my sis drove me nuts with it, on Cary Elwes movies I much prefer Men In Tights.
