Cancer. The worst types of it have the advantage of killing quickly. Alzheimer's destroys the self, and you survive a long time with it, leading to much more suffering, both to you (to the extent you continue to exist) and to your family.

I have a different perspective. The worst types of cancer kill slowly and cause agonizing suffering.

Alzheimer's leads to negative outcomes for your caregivers, but by many accounts many affected individuals do not suffer all that much, if at all, due to their lack of awareness.