6. The Wirrn (The Ark in Space)
Alright, we give Doctor Who some credit for not being afraid to go down a dark path. The Ark in Space in particular embraces a pretty frightening form of body horror, so much so that the BBC got many angry phone calls from parents whose little darlings were afraid to go to sleep. There's a tremendous buildup, where a character is infected and slowly becomes one of the Wirrn which is especially gruesome. But when he's done transforming and finally looks like a Wirrn...well, it's a bit of a letdown. It kind of just looks like a giant, German expressionist mosquito.