Realistically, I would say either 1 of 4 deaths:
- Being crucified. Death is excruciatingly painful and takes up to 3 days. The pain comes from having to constantly decide whether to put your weight on the nails through your feet or to hang from the nails through your wrists, all the while there is tremendous pressure just to breathe on your rib cage and diaphragm. Eventual cause of death is asphyxiation.
- Being burned at the stake by a slow fire. Death could take hours.
- Being drawn-and-quartered. You are hung by a noose around the neck until you pass out. Then you are revived, and they cut off your hands and burn them before your eyes. Then they cut off your feet and burn those. Then they cut off your genitals and burn those before you eyes. Then they hang you again. Then they cut you down and cut you into 4 pieces, and then finally behead you.
- Being cast into a well with a couple of feet of water in it. Death could from 1-2 weeks. While they is plenty of water, there is no food, and you cannot sleep for fear of drowning. Final cause of death is starvation, but sleep deprivation madness sets in long before.