Another creepy one is Kuru, a (now extinct) prion disease transmitted by eating infected human brain tissues. It's a very slow-progressing disease, and one of the last symptoms is commonly a week or two of "pathological laughter". Possibly not the worst disease to die of, but I bet it'd be damn creepy to see.
you mean they quit eating each other's brains in New Guinea?
vCJD is pretty similar, and that's still prevalent.
Sciagatt has a good point - Fatal Familial Insomnia is godawful.
If you're going to discount torture, those degenerate brain disorders are the worst. I'm not sure I wouldn't prefer to be suffocated in a silo full of flour than die of amyloid plaque brain degeneration.
especially if I already HAD amyloid plaque brain degeneration. in that case bring on the flour. make it whole wheat. bake a cake at my funeral.