Having a rat in a wooden box put on your chest. The side on your chest doesn't have any wood protecting it, so you can feel the rat scrabling around. Then someone sets fire to the top of the box. The rat can't escape the smoke and fire since it is in a box, so it goes the only way it can: it burrows throught you. How painfull is that going to be?
Also I haven't seen anything that really compares with being hung , drawn and quatered. Or a quaint method of tying a rope to each of your limbs, and tying each of the ropes to a horse, and then whiping them. You are then ripped apart.
Or in fact, being submerged in a back of acid, not too strong, so your body slowly disolves. It will take quite some time to die.
Or being trapped in a train, as smoke fills it up, with the doors jammed and fire almost reaching you. Death through burns or asphyxiation.
Or crucifiction. It could last hours, many many hours, as fluid filled up your lungs, the pain of nails in your hands and feet, you actualy had to pull on your hands and push on your feet to bring you up and stop yourself from suffocating.