According to an exhibit I saw (I think it was at the Amsterdam Torture Museum, or it could have been the Clink Prison museum in London), the most painful torture method was being boiled alive. I'm not sure how they knew this, maybe they boiled people and asked them how painful it was on a scale of 1 to 10?

Here are a few interesting sites about medieval torture (gruesome! don't read it while you're eating your breakfast)

http://www.livescience.com/history/top10_medieval_myths.html

http://medieval-castles.org/index.php

http://www.fidnet.com/~kawlaw/medieval.html

http://en.wikipedia.org/wiki/Torture

http://quizilla.com/.../



Footnote- I have made this post mostly about medieval torture because they were (IMO) the most imaginative when it came to inflicting pain. My eyes are watering just reading some of those links!