My first Pratchett book was Thief of Time (part of the Discworld series but further along, closer to 20th) -- I was absolutely stunned by his wit and humor. While I have read many other books of his (Discworld or not) and liked them and been amused by them, none other amazed me in the way Thief of Time did (perhaps because I now knew what to expect of Pratchett's style, of course).
Bottom line, if you think of getting into Pratchett, that is the very book I recommend. It did work for me (back when no one in my country, including myself, had never heard of him, so I had absolutely no idea who he might be, that he might be famous, that he might be appreciated, that sort of thing)
Later edit: the interesting thing is that, while I have mostly forgotten all the other Pratchett books I read, I can still remember not only what Thief of Time was about but also many many details of it