Cancer slightly wins out because I can't get over the existential horror of it being a potentiality any time a cell is damaged or replicates, but dementia to me is a cancer of the soul. It's one of those conditions where if I'm diagnosed and physically capable I'm walking from the neurologist's office to the roof. For every patient that seemed blissfully unaware of their condition, there were five who lived in abject terror of their own dying brain. Going through that for an indeterminate amount of years is something nobody except fascists deserve.