The hardest word in Hangman as of 2014 is "jazz." Typical challenging words in Hangman all have a few things in common. They're usually short and contain the letters "j," "q," "v" and "z."
Mathematician Jon McLoone calculated the hardest word in Hangman in 2010 by running millions of simulations with a computer. He ran each word in the dictionary through the simulation 50 times on the assumption that more common letters would be picked over the less common ones. McLoone then took the hardest 1,000 words and ran them through the simulation 3,000 times each. The results revealed the word "jazz" to be the word most often guessed incorrectly.