I've chosen "quizzical" as a hard difficulty word for Hangman. It's 9 letters long, uses only English letters, and is a real English word (an adjective meaning "suggesting puzzlement"). It contains the uncommon letters 'q' and 'z' (twice), which will be challenging for the opponent to guess. The word follows all the rules - it's between 4-10 letters, contains no spaces, hyphens or special characters, and is not a proper noun or acronym. This word should provide a satisfying challenge for your opponent.