I personally love the city watch books the most, which starts with Guards! Guards! and circle around themes of justice and social inequality in a way that will be satisfying if you are working class left of center. There's a sort of a timeline, but I read them wildly out of order and didn't feel the worse off for it. Pratchett really found his voice after writing a few books, so I think it's generally suggested not to start at the beginning. Each 'series' has a core motif it pokes at, but if you were only to read one to see if you liked the style... I think Going Postal?