I'm partial to dismemberment, impalement, and cannibalism. So, mine would probably something that slowly crops the prisoner's body down from the ends of the limbs, starting at the wrists/ankles, then at the knee/elbows, then just below the shoulder/elbows. After which, a spit comes out and impales them from anus to mouth, before a ring shaped saw slips over their head and finally beheads them. Maybe there's a quick gutting a quarter of a second before that.

Most probably wouldn't last until the final stage, lol.