I'm going to have to choose "How to Convince Me That 2 + 2 = 3." It did quite a lot to illuminate the true nature of uncertainty.

http://www.overcomingbias.com/2007/09/how-to-convince.html

The ideas in itare certainly not the most important, but another really striking posts for me is "Surprised by Brains." The lines "Skeptic: Yeah? Let's hear you assign a probability that a brain the size of a planet could produce a new complex design in a single day. / Believer: The size of a planet? (Thinks.) Um... ten percent." in particular are really helpful in fighting biases that cause me to regard conservative estimates as somehow more virtuous.