Let’s get the big one out of the way: publication order is king. Starting with 'Guards! Guards!' is non-negotiable in my book. I tried jumping into 'Night Watch' because a friend said it was the best, and I was so lost. You miss all the character foundations—Vimes’s development from a drunk to a commander, the introduction of Carrot and Angua, the whole dynamics of the Watch. The series is a long, beautiful arc, and reading it out of sequence just robs you of that slow build. Reading them as Terry Pratchett wrote them is the only way to catch all the running jokes and subtle callbacks.
That said, the Watch books are embedded in the larger Discworld, but they stand alone pretty well. If you’re absolutely new to Pratchett, you could start with 'Guards! Guards!' as a perfect entry point to the whole universe. The reading order after that is: 'Men at Arms', 'Feet of Clay', 'Jingo', 'The Fifth Elephant', 'Night Watch', 'Thud!', and 'Snuff'. Some people slot 'The Truth' and 'Monstrous Regiment' in there because they have Watch cameos, but they’re not core. Stick to this list, and you’re golden.