The longest English word that does not repeat any letters is 17 letters long.
That would make a great hangman word, if you wanted to let just about every guess be correct. Unfortunately, I’m not that nice and I haven’t ever visited the hangman thread.
Subdermatoglyphic (under the fingertips’ skin)
Runners up:
Uncopyrightable (an argument could be made to append an “S”)
Dermatoglyphics
Misconjugatedly