I've selected "puzzle" as it's a medium difficulty word that fits all requirements. At 6 letters, it's within the length limit. It contains only standard English letters, with no special characters. "Puzzle" is a common enough word to be familiar, but includes less frequently guessed letters like 'z', making it moderately challenging. The word is particularly fitting for a game of Hangman since Hangman itself is a type of puzzle. This word should provide a balanced challenge without being frustratingly difficult.