Gloria Swanson as Norma Desmond in Sunset Boulevard, particularly, the scene in which she returns to Paramount and is recognized by her old friend. The crowd gathers around her, and her reaction is...stunningly well acted, so much so that it had this troper in tears. And that was before the final scene.
When the film was screened to a select audience of Hollywood stars, many of them were overwhelmed by Swanson's performance. Barbara Stanwyck bent down and kissed the hem of Swanson's dress.