Personally I tend to advise people to read "Thief of Time" first but that's because it's gotten the most consistently good responses from the widest audience, in my experience. "Guards! Guards!" is also a good spot because it kicks off one of the best sets of characters in the series. I think that as you go back farther into his career chronologically, his jokes get more and more dry and British, and his references get less and less recognizable. I'd recommend "Thief of Time," "The Truth," "Guards! Guards!," "Pyramids," or "Mort," which is an older one but definitely really really good.