Impossible to say since once you die you can't tell anyone how painful it is.
Like the person above stated, it is impossible to tell. But I know about many ways in which some people have died that were painful to them. Such as the following:
- Cancer death.
- AIDS complications death.
- Labor and birth death.
- Having your throat sliced (not guillotine but with a knife).
- Being beaten to the point of brain trauma and swelling.
- Being burned alive.
- Heart attack.
- Brain tumor.
And many others along the line. Nobody can tell you which is more painful than the other. I assume most people wont want to go through any of those when they die.