This is a very often-debated subject… You'll want to tailor the choice to that person's taste.
Some novels have the same main characters as earlier ones, and they make more sense if you've read the earlier ones.
There's a reading order guide on L-Space , with a dependency graph by Krzysztof Kietzman ( also in picture form ).
Some of these dependencies are stronger than others, for example Wyrd Sisters and Witches Abroad make sense on their own, but Lords and Ladies is really a sequel to Wyrd Sisters .
The Watch novels are really a series, I don't recommend starting in the middle (so, not The Fifth Elephant ).
I'm partial towards The Colour of Magic (the first book in the series) myself, but
I know a lot of people don't like it.
It's a good point of entry for habitual fantasy readers ( The Colour of Magic starts with a Leiber ) spoof; Light Fantastic is a direct sequel.
Wyrd Sisters is a good introduction for a Shakespeare fan.
Soul Music for a rock-and-roll fan.
Witches
Abroad is built on fairy tales.
Small Gods is good for someone who likes philosophy of religions.
Mort introduces Death , and Guards!
Guards! introduces Vetinari
(the ruler of Ankh-Morpork ); both are endearing Pratchett characters.
Monstrous Regiment is good for a feminist.
Going Postal is set in Ankh-Morpork, but you don't need to have read the previous books to appreciate it; it shows the city confronted with modernity, 19th-century style.
Night Watch shows an earlier Ankh-Morpork in a revolution; it's almost independent from the other Watch books.
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"Guards!
Guards!".
Although I have other favorites, I think it stands alone well enough not to be confusing and require previous knowledge, but it is also a book in ther series where Pratchett has clearly honed his style.
You could also read the Guards/Vimes books sequentially after that too.
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For a younger reader---especially but not exclusively a young lady---consider Wee Free Men , the first of the Tiffany Achings novels.
My niece is sufficiently mannerly that receiving a thank you note was taken in course, but we heard through the grape-vine that she asked for the sequel, so I think it was a hit.
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First one I read was Mort .
But to be honest, if they're new to Terry Pratchett altogether, maybe go with Good Omens .
If they like the sense of humor there they should be good to go, and maybe pick up Gaiman as well.
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Two to consider are Equal Rites or Small Gods .
The former covers more of the reoccurring characters, but Small Gods is arguable Pratchett at his best commentary on Round World.
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I would always recommend Feet of Clay, I know its in the one of the watchmen series, but I find it's one of his most rounded books.
It has a lot of the running jokes, that are funny when you encounter them in the different books, but stand very well on
it's own.
I would also always recommend that you reread it after you've read a few of his other books.
I wouldn't start with the Colour of Magic, as I think that is better enjoyed by someone who's read a few of his books already.
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Pyramids, Small Gods, Wyrd Sisters.
Those 3 are probably the best introductions into the Dyskworld multiverse.
Small Gods might be less appropriate for a strongly religious person for all the obvious reasons :)
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For me it would either be the Thief of time or Mort.
I started with the former actually.
In my opinion, both of them plunge you into the extremely defamiliarising Discworld.
If you like the conceits and the humour, you'll know if you are sticking with Terry or not.
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The Truth is the first one I read.
It's after Pratchett's mainstays like the Watch or Death have already been developed but those books don't have to be read beforehand to understand who they are in relevance to the story.
It's very good at letting you get a taste of the series without being forced to jump into a whole sub-series.
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Start with the Rincewind cycle, continue with the early "stand-alones" (small gods.
pyramids) and DEATH cycle, then launch into the Watch cycle, then finish off with the witches...
but at least do the cycles chronologically, since it adds emotional connection with the characters.
My suggestion is to start where he did, even though RIncewind overall is a somewhat weak character (the other university people are much more fun in the later stories), by the time you get to the best parts of the Discworld saga (namely the city watch) you will know all about the (disc)world.
It's also fun to watch Pratchett develop his style from spoofing and light-hearted parody to the more bitter cynical humor of his later books.
Oh, and Vetinari and don't forget Strata!
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If you were going to loan one Terry Pratchett book from the Discworld series to someone, which book would you use?
Which Discworld book to start a newbie on
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<suggested-order><discworld><terry-pratchett>
scifi.stackexchange.com/questions/2416
scifi.stackexchange.com/users/1017/Justin C
Suggestions for an introduction to Discworld include The Colour of Magic; Wyrd Sisters; Soul Music; Witches; Abroad; Small Gods;  Mort  Introduces  Death; Guards!; Good Omens; Equal Rites; Wee Free Men; Feet of Clay; Pyramids; and Thief of Time. Very different opinions exist on which Terry Pratchett book should serve as an introduction to his works. Suggestions include Guards!, Wyrd Sisters, Pyramids and Small Gods, among others.
Very different opinions exist on which Terry Pratchett book should serve as an introduction to his works. Suggestions include Guards!, Wyrd Sisters, Pyramids and Small Gods, among others.