Radiation illness is one of them, there are certain yields that make it rather nasty. Rabies is also pretty high up the list.

Mechanically wise I remember the OSHA case of the worker getting trapped in a walk-in autoclave and literally steamed to death. because that actually takes a long time.

Prion diseases and locked-in-syndrome are also pure horror.

I guess the sandpaper room would be pretty far down on the list.

Where the F are serious mental health conditions? When your appearance seems intact from the outside, but inside your reality slowly yet steadily derodes, and there’s no way to help it. Going insane. That’s for me the last one. Prefer physical pain over losing touch with reality.