Drawn and quartered happens too quick. Saw it on faces of death, gruesome but when shock kicks in you don't feel shit.
They say freezing to death is the absolute worst because you are awake while it is happening. It's a slow process, and at the end you uncontrollably fall asleep. Of course by that time you have felt your entire body fail you.
I would also rate drowning very high on the list. Imagine drowning and gasping for air but inhaling water instead. Horrible way to go IMO.