I've been a big fan of Terry Pratchett's books ever since I first read the City watch Omnibus which consists of three books Guards! Guards!, Men at Arms and Feet of Clay. They were the first books that made me laugh out loud, and I mean REALLY REALLY loud.
After that, I bought almost every single Discworld book I could find, and some of his non-Discworld ones as well, and every year, the release of a new Pratchett book is almost as big an event as Chinese New Year (for me, that is).
Admittedly, I have yet to read EVERY SINGLE book of his, such as the The Bromeliad Trilogy (Truckers, Diggers & Wings)and The Amazing Maurice and his Educated Rodents, but I'm GETTING there.
Nevertheless, of the ones I've read, here is a list of my favorites, with a short review of WHY I liked them so much:
1) The City Watch Omnibus (Guards! Guards!, Men at Arms & Feet of Clay)
Ok, so this is actually THREE books, but when read together, the reader gets a great introduction to the City Watch characters, starting off with Vimes, Colon, Nobby and Carrot in Guards! Guards!, then adding Angua & Detritus in Men at Arms.
To me, Feet of Clay marks the first book where these central characters REALLY gel together, while Vimes and Carrot begin to come of age after two books of growing up as characters. After this book, the subsequent City Watch books worked better because the characters were already so well-established.
It also happens to be the first Discworld books I've ever read, so these books have a special spot in my reading heart. heh.
2) Wyrd Sisters
Although Equal Rites introduced the indomitable Granny Weatherwax, she is at her best when supported by her coven (Nanny Ogg and Magrat), and Wyrd Sisters is the first book in which they come together. After that, the Lancre witches are as prominent in the Discworld universe as a wizard's knob in a nudist beach (Go figure).
3) Reaper Man
Death is one of my favorite characters in Discworld, and it is no surprise that three books on this list feature him as the main character. Reaper Man is the first book that Pratchett uses him as the lead character rather than a mere supporting one like in the other books. The premise is brilliant (what would happen if Death decided take a break from his job?) and the Death of Rats (SQUEAK!) is one of Pratchetts funniest creations yet.
Death as Santa Claus. HO. HO. HO. 'Nuff said.
5) Monstrous Regiment
This book is one of the more 'serious' Discworld stories written (though still darn funny), and probably the most thought provoking, as Pratchett explores the dark, dark world of war, feminism, racism, and the consequences of caffeine addiction.
6) The Wee Free Men
Ach! Crivens! This is supposed to be a 'children's book' but adults would probably enjoy this more than kids. It is hilarious. Of course, what would you expect from a book about a race of 6 inch tall blue men with red hair and a fear of lawyers? Look out for the sequel - A Hat Full of Sky, coming out soon.
7) Thief of Time
I LOVED this book mainly because of the Four Horsemen of the Apocalypse, and The Fifth Horsemen (who left before they became famous). Besides that, the story flows along wonderfully, and the characters are the type that you really wish you could meet. ALSO, books about mucking around with Time always guarantee a good non-waste of time.
8) Soul Music
Death and Rock n' Roll. What else can I say? This book parodies the music business, turns good old rock n' roll into 'Music with Rocks In It', and turns out to be one of Death's most entertaining outings.
9) Interesting Times
Even though Pratchett started out the Discworld books with Rincewind as his main character, he quickly discarded the inept wizard after the first two books (The Colour of Magic & The Light Fantastic) to focus on more interesting characters. Personally, Rincewind is my least favorite of the Discworld main characters, because he was so annoying (though The Luggage is one of my favorites).
However, Interesting Times is er.. interesting because of the fact that it parodies China, has the Luggage running, and Cohen and his band of senior citizen Barbarians (Hur, hur ,hur). One of the funniest in the series, IMHO.
10) Moving Pictures
One of the few Discworld books that doesn't have either Rincewind, Death, the Lancre Witches or the City Watch (although Seargent Colon and Nobby DO make a cameo appearence) as the main characters. The book parodies Hollywood, and ends with whats has GOT to be one of the funniest spoofs of King Kong in a book or film EVER.