What is the hardest riddle?

According to Business Insider, the so-called 'hardest riddle available on the Internet', Notpron, lives up to its name. They reported that, as of September 2014, only 31 of the more than 16 million people who had attempted to solve the riddle had actually succeeded.

The riddle was created by David Munnich as a web browser game. It has 140 levels of gradually increasing difficulty. Although it makes hints available to those attempting to solve the riddle, as well as a discussion forum (both free of spoilers), it is nevertheless almost certain to stump most — the success rate being around 0.0003 percent.

Success requires creativity and knowledge of computers and the Internet.