Flayed alive.

Having your skin removed like you're some rabbit for the stew, a cut is made, then the knife is slid under the skin, separating flesh from the skin, the skin is then peeled from you whilst you sit there in agony forced to watch as they start with the lower extremities and work their way up. first a toe, then a foot and a leg. If they do it right, they'll keep you alive long enough to be awake whilst they remove the skin from your body right up until they reach your chest, perhaps higher. Or they may work their way around your body in the most degrading or painful of ways that keeps you alive the longest.

Being flensed like that has got to be the worst way to die.

Little known fact: Historically records show that some people burnt at the stake would have a small pouch of black powder (and iron pellets or the like) fitted to their person near the heart. As the fire gets hot enough eventually the black powder would explode. When it did the suffering would be cut short by what was tantamount to a buck shot round to the heart.