Is the CPA exam the hardest professional exam?

As a CPA, I would say that it is difficult to judge something like that, because everybody's testing experience is relative. For example, I found the CPA exam to be difficult, but not nearly as tough as my peers led me to believe it would be. However, the pass-rates are less than 50%... I'd say this is most likely to both the quantity of the material, and the fact that most people studying for the exam are working long hours at full-time accounting jobs.