Top 10 most terrible fatal diseases

I'd have to nominate fatal familial insomnia to the list. Fortunately it's very rare, but not too rare to be mentioned in our behavioral science text (p. 109). Basically, you can't sleep AT ALL, and end up dying after up to a year and a half of hell. Prion disease, autosomal dominant, age of onset around 50... more info at Wikipedia. I first heard about it on NPR a couple days ago.

Any other nominations? This could be interesting...