I think the first book or most of the first few books are not necessarily the best starting point, since it takes a while for the universe, the tone and the comedy to gel. The first two in particular are quite different from the rest of the series—much more a direct fantasy parody. I actually enjoy them a lot, but they're not representative. It's better to go back and read them later—then you get a sort of The Magician's Nephew prequel effect of seeing how certain things came to be (notably how the librarian became an orangutan).
I'm in the middle of a re-read right now (literally the middle—I'm halfway through Jingo), and I would suggest Guards! Guards! (no. 8) as a starting point. Mort (no. 4) is also commonly recommended, but I personally prefer Equal Rites (no. 3), even if it does have some early-installment weirdness.
Are you looking mainly for comedy, fantasy worldbuilding, social satire, moral philosophy, coming-of-age fantasy stories, or what? Different Discworld books cater to different tastes.
I strongly disagree with the author about starting with Sourcery. It has some interesting bits, but overall it's one of the weakest books in the series.