A good one-off book is Good Omens, which Pratchett co-wrote with Neil Gaiman. Very funny and at one time Terry Gilliam was going to make it into a film (there's a Monty Python connection for you
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) but he went off to make the Quixote film instead. I think other directors are now looking at it.
As for Discworld there's several options. Reading the whole series in order is the logical way to go, although the earlier books aren't as good as the middle ones and some of the newest ones. The first book, The Colour of Magic, is very different in tone and style from the other books, although it is still funny in parts.
The advice I usually give is to start with the City Watch books. These books have a continuing arc of sorts and the character development of Vimes may be Pratchett's finest achievement in the series. They're also pretty funny and, being a fantasy take on police procedural dramas, very approachable. The first one is Guards! Guards!, followed by Men at Arms, Feet of Clay, Jingo, The Fifth Elephant, Night Watch and Thud! Vimes also has a supporting role in The Truth and Monstrous Regiment.
The one-offs with no recurring characters (or only recurring characters in background cameos) are also good to get a feel for the way Pratchett writes: Moving Pictures, Pyramids, Small Gods, The Truth, Monstrous Regiment and Going Postal are all good in this regard. Small Gods is almost certainly the single best thing Pratchett has ever written.
I'm not a big fan of the Granny Weatherwax/Lancre Witches plot line, but some people swear by them. These books are Equal Rites, Wyrd Sisters, Witches Abroad, Lords and Ladies, Maskerade and Carpe Jugulum. Of these Lords and Ladies is by far the best.
The Death books are pretty dull IMO but again they are well-regarded by others. These books are Mort, Reaper Man, Soul Music and Hogfather. Mort is regarded by many as Pratchett's best novel, but I found it pretty bad.
Finally the Rincewind books are good for a knockabout, Monty Python-esque laugh: The Colour of Magic, The Light Fantastic, Sourcery, Eric, Interesting Times and The Last Continent.
Then there's the Young Adult Discworld books: The Amazing Maurice and His Educated Rodents, The Wee Free Men and A Hat Full of Sky. The Wee Free Men is currently in development as a movie as well.