While it is impossible to say which is the hardest riddle, an example of a very difficult one is: "Ten men's strength, ten men's length, ten men can't break it, yet a young boy walks off with it." The answer is a rope.
Riddles are typically split into two categories: Enigmas and conundrums. Enigmas usually use metaphor or allegory to make the listener think abstractly. A great example is the Riddle of the Sphinx from Oedipus Rex: "What walks on four feet in the morning, two feet in the afternoon and three feet in the evening?" The answer is a man. Conundrums require puns to mislead the listener, such as "What gets wetter and wetter the more it dries?" The answer is a towel. A number of very difficult riddles like these can be found on websites such as Riddles.com