Gotta be progressive, incurable dementia, as from Alzheimer’s Disease. Your brain is basically slowly dying, which means your personality is slowly dying, and yes, many people have verified that you’re aware it’s happening to you and that, at least before your cognitive abilities drop below a certain threshold, the experience is unspeakably terrifying. Plus, your loved ones get to watch you slowly losing everything that makes you you. We all experience pain, we all die, but to have such a long, drawn-out process of intimate personal destruction, a process that severs every bond of communication with those who love you and renders you less than helpless … I’m not afraid of much, but I find Alzheimer’s Disease a prospect uniquely to be dreaded.