I would second this! Going Postal was the first Pratchett book I read and I think it’s a fantastic one to start with, because you don’t need to know much of anything about the world and the plot fires on all cylinders all the way through.

I know the person who always recommends “guards guards” posted first here, but I find that one of the weakest books in the series for what it’s worth.

The thing is… It sort of depends what kind of books you like. I’m not crazy about murder mysteries, so the guards books don’t do much for me. I love satire and fantasy storytelling, so Carpe Jugulum and Lords and Ladies are faves. I love opera, so right now I’m re-reading Maskerade! If YA fantasy is your thing, the Tiffany Aching books are perfect.

For my money though his best book is small gods, which is an utterly humane satire of all human religion (and atheism). If you were going to read one, I’d say that one – but it’s very much a standalone and an introduction to his writing style more than it is a Discworld novel.