My favourite Terry Pratchett books are Soul Music which introduces Susan Sto Helit - probably my favourite Pratchett character - Interesting Times in which the 90 odd year old Cohen The Barbarian attempts to conquer an empire using a bare handful of old men, and Men At Arms in which the Ankh Morpork City Watch get a few rather interesting new recruits...
As a general rule though, you can't go wrong with anything with Susan or the Ankh Morpork City Watch in it.
All the Discworld novels are at least readable, and many are utterly superb, but Susan, Carrot and Sir Samuel Vimes are Pratchett's strongest characters, IMO.
If you want something non-Discworld related, Good Omens is an excellent (As recommended by DATD, above) standalone.
As you are a Gaiman fan, I'd love to hear your thoughts on comparisons between Harry Potter and Tim Hunter. Coincidence? Or not?