I would personally start with either Guards! Guards! or Wyrd Sisters rather than the wizard books, but that's because the wizard books (plus Equal Rites) are the earliest in the universe and are thus a bit thin compared to the others. (I also find Rincewind annoying and think he works best in small doses.) Pyramids, Moving Pictures, and Small Gods are also good starting points if you just want to get a feel for the world and Pratchett's sense of humor. (The Death books and the Industrial Revolution books both work better if you already have a sense of how the Discworld universe is set up. (I don't count Moving Pictures as an Industrial Revolution book for, um, reasons that are a complete plot spoiler, now that I think about it.))

But really, you can start anywhere! The books are all generally capable of standing on their own. :-)