I loves me some Pratchett. I gobble up these books as fast as the man can write them.
Thief of Time is excellent (I would definitely second a nomination of the Sweeper for Discordian Sainthood - we need a copy of his notebook!) and I'm also very partial to Night Watch, a great satire on revolution. Small Gods satirizes philosophy and religion wonderfully as well. He does a great job of poking fun at whatever he feels like without slamming his point over your head like a brick to the face - and makes you laugh while he's doing it. Highly recommended.
You Brits are lucky to get the tv shows in addition to the books over there - I can't wait to check out that Hogfather stuff. I'm definitely curious to see how an ANTHROPOMORPHIC PERSONIFICATION gets portrayed - who knows, maybe they pull it off.
The good news is the Death of Rats made it in there and he's totally true to the books!