That's why psychological torture is preferred by a lot of "advanced interrogators".

Keep someone locked in the same uncomfortable position for days while you're blasting aggressive disorienting sounds and randomly flashing blinding lights in a room with mirrors.

Once in a while, take them and do a few hours of water boarding. Make sure they're constantly sleep deprived and can never get more than a few minutes of sleep.

Then when you do want to inflict physical pain, focus on the feet very slowly. Feet is one of the most sensitive areas. Slowly start peeling the skin or stick nails through the toes, or practice some electroshock therapy.

Honestly torture is terrifying. I recently read a long form article about it. Worst part is, our "civilized" governments still do this with some amount of regularity. Would be more civilized to put a bullet in the head.

Then think about near future where we can read thoughts (like 1984 where the "advanced interrogator" reads the protagonists mind to figure out what his greatest fear is) or even worse implant thoughts. They could implant images of you murdering your family or something. There's a lot of potential for some horrifying stuff.

That's why in some science fiction, see Warhammer 40K, they have technology that let you keep on feeling that pain long after you should be well and thoroughly dead. Honestly I'm not sure who's better at it, the Imperium or the Dark Eldar.

So perhaps the most painful death would be just before that threshold.

I've seen some nasty videos I'd rather forget where they scream and cry all the way through it.