My favorite strategy: four letter word. Say no to everything until they guess U: _ U _ _ then decide on the least likely word which includes none of their guesses, often "junk".
I cheat the same way by selecting "_OO_". Guessers get frustrated because, intuitively, it doesn't look like there are 30+ english words matching this pattern: root, fool, loom, noon, etc :-)
If you play that way in Dutch, you can use three letters and even give away that the word ends in 'ak'. That leaves at least BDGHJLMPRTVWYZ for the first letter (A and K give valid words, too, but will already have been guessed)
In general, shorter words are harder to guess in hangman.
