Jingo.
I love all the watch books but there is something special about Jingo, the dis-organiser / trousers of time, Nobby / Colon / Patrician, the incredulity Vimes felt at the actual 'Planning' of a war, Vimes's butler, etc etc...
I feel that this book in particular is a rich tapestry of plot, story and character, and it loses none of its sparkle when I come to re-read it(which I do more often than is probably healthy :? )

I could probably say the same sort of thing for most of the books, but to me, Jingo will always be slightly above the rest.

Cheers.

R.