That guy that got acid in his face in AVPR was one of the more cruel deaths I would argue.
Most of the ones mentioned are also cruel, but to me the worst one isnt a human but a xeno death:
The queen alien in resurrection had probably the worst death and life: going through its mutation and pregnancy, to endure the newborn bursting out of her womb like a chestburster but taking way longer and more painful, and after that long ordeal the thing then swaps off her head and she die slowly.
To me she had easily the worst death.
I laugh when people say they are sad for newborn, to me it got what it deserved and for being so stupid, that I laugh at it dying. xD