So Discworld is made up of a lot of smaller series of books. Many characters and things will cross series, but each has it's own feel.
Personally I'm a big fan of the Witches series. It technically starts with Equal Rites but you could also jump in with Wyrd Sisters.
I also think Guards! Guards! is a solid entry point. And you can never go wrong with Mort. It all really depends on what you want to read about.