OK, I'm going to go out on a limb here, and say that "Night Watch" is Pratchett's best Discworld book yet.
Why? Because the characters in the book have been so long-established in the Discworld universe that it is uniquely revealing to see their younger selves in the era in which Sam Vimes began his career. This is a bit of a quandry, because in order for this book to be so good, it needs the reader to have gained an understanding of these characters by first reading the other City Watch books in the series.
"Night Watch" is, as noted by other reviewers, also darker in many ways than the other books of the series. There is sadness here, and grief, and heroism, and horror. This mix of humour and darket tones works extremely well, especially as the sadness is interwoven seamlessly into the overall plot and character development.
In all, it makes a refreshing departure from the other (still excellent) tales of the Discworld.
I'd be very happy to read more books of this hue from Terry Pratchett. It is especially welcome bearing in mind that the Discworld series is now approaching thirty books and shows no sign of becoming repetitive, stale or reduced in humourous content, or slowing down for that matter. A tribute to Mr. Pratchett's seemingly bottomless well of talent, I'm sure.