Over the course of a season, Donna Noble went from just being a temp from Chiswick to being a well-rounded, confident person. Shed moved on and had some of the most intense character development of anyone ever on Doctor Who. Then, in her final episode (more or less), all that character development was lost, and she was kicked back to square one as her memories were erased. Heartbreaking doesnt even begin to describe it. Infuriating just about does, as I recall being very pissed at what the show did to her. Even now, still annoys me. This one wasnt perhaps especially heartbreaking for the characters, but it certainly is for many, many fans. Honorable mention: The climax of To the Death and the death of Adric.