My favorite Watch book is Fifth Elephant, followed closely by Feet of Clay.
You know, I love the watch books. Feet of Clay was the first TP book I ever read, the next one was Gaurds! Gaurds! and I loved them both. But I could not get into Night Watch. I was so excited for it to come out, and when I finished reading it, I put it down and went "huh". Most of his books I read more than twice because there are things I missed before and there are really great stories. Night Watch, however, I have only read once and really have no desire to read it again. I cant even really put my finger on what I didnt like about it. :?
But, I digress.