Damn, it's hard to pick just one, but if I had to I'd choose Theif of Time. It's great. I want to learn Deja Fu.
But I love Rincewind and the Luggage, same with DEATH and his granddaughter, Susan. They are all great characters.
Oh, and who here has read Good Omens? That is equally good, though it's set on Earth instead of Discworld.
Last thing...I was casually watching anime on cartoon network, and they were showing the Gundam series where the fighters all represent different countries, and they fight in this big competition for leadership. (I don't remember the name of the series at all.) Well, in one of them they start showing different planets/creations, and I saw they had included Discworld. Giant disk on 4 Elephants who stand on a giant turtle. I started laughing when I saw that.