Anyone who tells you there's one perfect order is kidding themselves. I've been through the series twice now, in two wildly different ways. The first time, I followed a popular 'by sub-series' chart I found online, starting with 'Guards! Guards!' to follow Sam Vimes. It was a solid intro, but you know what? I felt like I was missing inside jokes about the Unseen University and Death when they popped up. My second read was purely chronological, starting with 'The Colour of Magic'. It's rough around the edges, but there's a charm to watching Pratchett's world and style evolve from the very first sentence. It made later books feel like reunions with old, polished friends.
Honestly, my best advice is to pick a starting point based on what you normally like to read. Love police procedurals and city intrigue? 'Guards! Guards!' is your book. Prefer coming-of-age tales at a magical school? 'Equal Rites'. Into fairy tale satire? 'Wyrd Sisters'. The connections between the sub-series are more like gentle winks than required reading. You can absolutely dive into any of those and have a complete, wonderful time. The worst thing you can do is get so paralysed by flowcharts that you never actually pick up a book. Just start anywhere except maybe 'The Light Fantastic', since it's a direct sequel, and let the Disc pull you in.