My vote would go to crucifixion, even if you don't belive in Jesus or the Bible crucifixion was actually practiced and it is alot more painful than most people may think. here is a brief explanation of what goes on. I think you'll find it interesting.
It all starts with scourging which would weaken the victim, cause massive blood loss, and probably induce shock. By the time the victim had carried the cross bar to the crucifixion area, he would be exhausted. (yes victims actually had to carry part of their crosses to where they were going to be killed.)
Once up on the cross, the victim would have his body weight suspended by their arms. In this position, it is difficult to completely exhale. The victim could take shallow breaths for a while, but eventually would be forced to push himself up to take a full breath. (keeping in mind that both the hand and legs are impaled)
At this point three things happen:
The victim's weight is now fully supported by his feet. The nails through the feet hit two major nerves running through the area. The result would be excruciating pain in the legs.
The nails in the wrists would pierce the main nerve running through the arm. As the victim pushed up to breath, the wrists would rotate against the nail, irritating the nerves and causing intense pain in the arms. Some authorities also believe that the crucifixion position would dislocate the shoulder or elbow. Any movement would aggravate the pain from these injuries.
The wounds on the victims back from the scourging would push up against the rough part of the centerpiece. This would tend to re-open the wounds, leading to more pain and blood loss.
This combination of pain would quickly force the victim to lower himself back down. Eventually, the victim would no longer be able to raise himself up and would slowly (very slowly) suffocate to death while slowly bleeding and in constant pain. It could even take days for the victim to die.
not exactly a nice way to die. Along with crucifixion I think that being in a nazi concentration camp would be the worst way to die.