Being tortured in any painful way and then getting treated by expert doctors so that you can be tortured again. Repeat process until dead. (I don't think anyone could last long, even with medical treatment.)
If I truly wanted to tell you the worst way to die, I would. But this website has a pg13 community, so I'll keep my mouth shut there. A pretty bad way would be torture till death with your family and loved ones having the same treatment in front of you. Death of a 1000 cuts.