And if you don’t, that doesn’t mean you won’t like the rest of the series.
I think I’ve read every one of the Discworld novels (and most of them multiple times), except Colour of Magic. Started it several times and never managed to finish it.
I’d say Guards! Guards! is the best place to start. I feel it’s a much better book than the earlier ones, and also important for understanding many of the characters that recur in subsequent books.
Let us not forget Bestiality Carter however. 
Thirding, fourthing, whatever. These two books are the first installment in each of two sub-series, namely the City Watch and the Witches and between them lay many of the foundations seen in later Discworld novels.
Haven’t read I Shall Wear Midnight yet – loved the other Tiffany books though. Thud! didn’t feel formulaic (to me), and it’s a book I’ve enjoyed more each time I’ve read it – I’ve found there are some Discworld books that really didn’t grip me the first time but that on re-reading have provided far more enjoyment…Monstrous Regiment is another of these for me.
Unseen Academicals was on the other hand a huge disappointment… it felt to me like a Pratchett in name only… something ghostwritten for him… and most unsatisfying after the hugely enjoyable Nightwatch and Going Postal.
I actually had Wintersmith and Making Money in mind - I agree that Thud! was good, but I haven’t read Unseen Academicals and I Shall Wear Midnight yet.
Making Money fell totally flat for me. Moist von Lipwig was a one-novel character at best. Thud! was great. Unseen Academicals was somewhere in the middle, mainly because I think football is stupid.
I recommend listening to these books. Nigel Planer does most of them, with Steven Briggs reading from around The Fifth Elephant onward, and he suits the shift in tone that comes with that book very nicely. He doesn’t have that Asterix and Obelix attitude towards them that Nigel Planer does. That’s how I see the denizens of Ankh Morpork and Lancre: they all look like characters from the Asterix books.
Personally I think Terry Pratchett struck gold (again) when he came up with Moist von Lipvig. He’s one of my favourite Discworld residents.
I constantly confuse Thud! and The Fifth Elephant. Thud! just seems like an “improved version” of the latter. Pratchett tends to do that a lot. Which is why you should read the books in order. When I started reading the series I made the mistake of reading Reaper Man before Mort, big mistake.
Well, Equal Rites is sort of a witches book, and comes before Wyrd Sisters (at the very least, both feature Granny Weatherwax). But Rites doesn’t really seem to be in continuity with all the rest.
Recent events have brought it further back into continuity.
Ah, I’m only as far as Going Postal, so don’t spoil me with details. I’m a bit surprised, though… I would have thought that, had he wanted to tie it back in, he’d have done so sooner.
But it does depress me a bit that one or two more Amazon orders, and I’ll be caught up. I mean, then I’ll have to wait for the next release, like everyone else, instead of just being able to order more whenever I want!
Night Watch was actually the second Pratchett book I read. Well, the third, if you count Good Omens.
I was a fan of Neil Gaiman, had never heard of Pratchett. Found Good Omens. I could tell which writer wrote which parts, it seemed glaringly obvious, but I liked the other writer, Pratchett, enough to try one of his. Small Gods was the first real Pratchett one, and yep, I went to Night Watch right after.
It remains my favorite, though of course it just grew better after having read other Night Watch books and learning a bit more history.
I still think it’s the best novel. >.<
ETA: Hey!! Someone do a Pterry poll!
Favorite books, favorite sub-genres, whatever.
I read the series as they came out in paperback.
Colour of Magic was fun because Sir Terry set out to lampoon fantasy fiction tropes. (The names of the dragonriders, for example, come from Dragonflight.) He wasn’t telling a story as such, just stringing some jokes together.
The next few books were somewhat funny, but not as good as COM; this is because they were actual stories, but Sir Terry wasn’t into his stride. IMO, he started to hit it with Sourcery. Once we got to Wyrd Sisters, Pyramids and Guards! Guards! he was off to the races.
My favourites are definitely the midperiod books, but most have something to recommend them. Of the recent book, Unseen Academicals is the one I like least, and I can’t help wondering if that is due to his illness.
I thought Pterry was simply declining in quality but he seems to have hit some kind of sine wave where some books are great and some are drivel. I like his earlier stuff but then I started reading them when I was about 11 back in the late 80s and have been following him ever since, so it’s a bit hard to be objective.
I agree that starting them out of sequence is a good way to get into him, and I’d probably go for Mort, Wyrd Sisters or Small Gods too.
One of his books (I don’t remember the title) struck me as being a rewrite of Tove Jansson’s book Comet in Moominland and when I asked him if he had ever read Jansson (“of course” was the answer) and told him about the similarities he seemed genuinely surprised. The obvious explanation must be that he had read it as a child and the basic story had lingered in the back of his mind long after he had forgotten about it.
I take it not as a plagiary but an homage.
Me too - I found for example the list of titles joke puzzling, pointless and drawn out and the whole goblin origins plot heavy-handed and preachy. Some parts of the book just seemed off. 
He seems to be preaching tolerance in many of his newer books and while it is a commendable message, it can get old.
I started with Reaper Man, and I’m glad I did. It really drew me into the series.
I haven’t read Unseen Academicals yet.
I think that the two Moist Lipwig books (Going Postal and Making Money) were fairly recent, and I loved both of them. Going Postal in particular is one of my all-time favorites. (And not just because I’m married to a mailman. 
)
Quoth Taomist:
I’ve heard the two of them say in interviews that the parts that were obviously written by Pratchett were actually Gaiman, and vice-versa. They were deliberately imitating each others’ styles.
OMG, now that I know this, I’m going to have to get more familiar with both of them, and read this!
For me, one of the great pleasures of reading The Talisman (Stephen King and Peter Straub) was picking out parts that were obviously written by one or the other.
In this light, Good Omens sounds like so much fun!
If I liked Small Gods very much, but was insufficiently interested in either Guards! Guards! or the other volume I tried to even finish them–that doesn’t bode well for trying any others, does it? The others didn’t seem to have any ideas comparable to Small Gods’.