As others have said this is a hard choice, since the Watch books are my favourite in general. Guards, Men at arms and Feet of Clay particularly, hmm...I have to go for [b:27dba23ea7]Men At Arms[/b:27dba23ea7] but it is a close call. I like how the watch has to take on new recruits and Detritus and Cuddy are funny together, Gaspode gets an outing and Carrot meets Angua.
The story flits between all the characters and their part in it brilliantly.