The wheel was always my favourite. Break the arms and legs in enough places that the limbs lose stiffness and can be woven around the spokes of a large wheel such that the victim is spreadeagled. Usually requires the ankles, knees, hips, wrists, ankles and shoulders to be shattered. Once the victim is secured, raise the wheel up atop a post so they are facing the heavens. Death by exposure, provided they survive the bone shattering.
I read a short story once that included one person being executed by having molten silver poured in the eye and ear cavities before being dumped in a deep pit and left to die over several days, blinded and deafened, but I don't know how realistic survival of that is.
Crushing is fairly brutal... tie the victim down, between wooden doors, and start slowly piling boulders on top until their chest collapses.
Death by wild boar dinner.
Lashed into a cow-shaped device and wheeled out to the bulls in the field.
Anything suitably slow, painful and needlessly cruel can work.
Oh! Almost certainly fictional, but have you considered the bloodeagle?
Oh! Enclosing in an iron cylinder and roasting to death.
What was that one they used to do with living bamboo?
