Reginald Shoe would be so happy to see this thread dragged un-alive and kicking from its musty tomb.
but because I can I will repost what I wrote in 2007 with slight amendments
1 I recommend that you DO NOT start with The Colour of Magic or its direct sequel The Light Fantastic. These are the first two books but don't quite have the same flow or depth that would define the rest of the series and thus Rincewind is my least favourite character. (In my opinion it's not until the fourth book Mort (the first Death Novel) that Pratchetts Discworld style really starts to shine.)
2 Regarding Ricewind later books "Interesting Times" and the "Last Continent" are much better, so you could start your Rincewind and Twoflower journey there.
3 The Watch series is certainly the most accessible (although it has morphed slightly from being about the Watch to being about the Life of Samuel Vimes). Many consider Night Watch to be Pratchetts masterpiece, although THUD could also claim that title.
I'll sneak Monstrous Regiment in as Watch-Adjacent though it's not about the Watch at all (it is a good meditation on propaganda, nationalism and gender roles in a war comedy wrapper.)
4 The Death series is Pratchetts chance to get philosophical and make observations on the human condition
Susan Sto Helit is part of the Death series but should have got her own series (sadly Thief of Time was the closest we got to that, although Hogfather is a Susan story too)
5 The Witches series gets better over time and requires pantomime sensibilities to really gel. Of the Witches series Lords and Ladies and Capre Jugulum are imho the best.
6 The Tiffany Aching series is also fun, though it fits on the children's literature shelf with The Amazing Maurice and his Educated Rodents.
7 Moist von Lipwig is a later character (first appearance 2004) who got a trilogy that helped modernise the Discworld into the modern industrial age.
Unseen Academicals also fits in this set though it does not feature Moist von Lipwig.
8 The stand alones are Small Gods - which is a quintessential package of all of Pratchetts Discworld philosophy - and Pyramids, Pyramids which really is an isolate that never shows up again, except in the Djelibeybi joke
9 I also recommend you read Nation, its technically not set on Discworld, and may infact be 18th century Earth, but it is written in Pratchetts discworld style and is a great story too