That scene was so amazing. WHERE'S MY COW! :D
I think Night Watch is maybe my absolute favorite - I've read it three or four times, and it still gets me swinging between hysterical laughter and sobs (which is my kind of book :)). That was actually the book that got me hooked on Pratchett to begin with - I'd read a few of the earlier Wizard-focused ones, and they were okay and amusing, but not astounding or anything. I'd say Night Watch (and some of the other Watch books, like Thud and Jingo) are astounding. I think I also like that they tend to deal a bit more with questions like justice and such, but in typical Pratchett style.
Other than the Watch books, I think the Witches books are my second favorite (including the Tiffany Aching series). Granny Weatherwax kicks some major ass - I aspire to be that kind of old lady when I grow up. ;) Plus, what's so satisfying in the Witch books is how Prachett manages to make fun of "fluffy bunny" types while still showing enormous sympathy and subtlety for those characters. It's those kinds of books that make me think he must have at least a few Pagan fans keeping him in line and letting him know what the real stuff's all about. ;)
The Wizard books tend to be more funny, and the Death and Witches and Watch ones (post-Reaper Man, anyway) tend to be more serious, with the Moist ones somewhere between. I must admit, though, when it comes to the one-offs Small Gods is the exception to the rule; most of them are nowhere near as good. I kinda liked The Truth, and even so have to admit that (The Patrician/Tyrant of Ankh-Morpork) Vetinari's small roll was the highlight of it. Well, that and the vibe of Ankh-Morpork, and the feeling of walking around the place as a stranger to it.
Although the Luggage, Rincewind, Death, Carrot and Vimes sometimes give him a run for his money, none of them ever quite edge out his exquisite awesome. Oh, and the Feegles, of course, but everybody loves them. I think if he were able to write enough more of them for us all to really get to know them, Moist and Adora Belle would be up there, too.
So, Hogfather and Reaper Man for Death, Feet of Clay and Guards! Guards! for the Night Watch, and Carpe Jugulum and Maskerade for the Witches.
I like me some Terry Pratchett stories.:)
Someone already mentioned it, but Good Omens (co-authored with Neil Gaiman) is probably my favourite book of all time.