Q. X, Y & Z are the four main ones. Each of those four have very few corresponding words in the dictionary. Next toughest group to sell because of limited word choices are J, K, U & V (German dealers will point out that K is good there as it if often used where the English language uses C, however I've never had much luck with them with that more limited market).
From analyzing various English letter frequency web sites, I have determined that the worst letters in the English language are j, k, q, v, w, x, y, and z, but keep in mind, jfk.com is probably worth quite a bit. :wink: