When I used to work with metal, there was a guy, who on his first day got pulled in to a grinding machine. He screamed for about 2 or 3 seconds, then went quiet while what was left of him got chopped up and spat out the other side of the machine.
Something like this would get my vote, the reason being, as soon as it happened, I looked at the mince flying out of the machine and knew instantly he was dead. The first thing that crossed my mind, was “that totally did not need to happen”. It was a careless and needless death, a big waste, avoidable, and therefore the worst.
If we are talking pain wise, then it would have to be getting tortured to death, as the process could be made to last for months or even years perhaps, and could involve a little of everything. It would all be down to the torturer’s imagination.