Drowning

Knives, disease, war, parasites, none can compete with the simplicity of being trapped underwater until you die. It takes a good while to lose consciousness through lack of oxygen, so you have a good long while while you're drowning to know what's happening, and what's likely going to happen next. You also won't be able to see or hear anything, meaning all your energy is going to be used focusing on the fact you can't breathe, and that slowly but surely your brain is shutting down piece by piece. My commiserations go out to all the unplaced competitors, as well as anyone who's ever drowned, which we've established is a terror unlike no other. Sincere congratulations to drowning, which has rightfully topped the most horrible and depressing poll of all time.