Historically I'm thinking flaying. That is removing the victim's skin while still alive. I read that during the "peasants war" (baurenkrieg?) prisoners would have the skin removed from one of their arms, from the shoulder down. They were then left to die from shock (if you're lucky) or infection.
In modern times I think the "Joberg necklace" is right up there. Force a tire filled with gasoline or diesel over the victims shoulders and light them on fire. Anyone who's ever tried to get the water out of an old tire will understand. The molten rubber and gasoline mix is just like napalm. The victims agonised movements just spread it around. The worst part is it doesn't always kill right away, leaving a still breathing (barely) victim with all of the flesh burned away from their skull, chest and upper extremities. The most cruel and brutal way to die I can think of.