Damn. He was fading fast, but it's still sad to see him go. I only recently started reading the Discworld series, which is an absolute hoot. Just a crazy invented fantasy world, with really colorful recurring characters. Frequently hilarious. The first one is The Colour of Magic, although I gather you don't have to read them in order.
I also really enjoyed Good Omens, his collaboration with Neil Gaiman.
The Discworld stuff which I was first introduced to when I saw a BBC special movie featuring one of the stories. I think the quote I remember and the one that got me started reading the books was from death. He said about humanity "In a universe of infinite wonders you have managed to invent boredom".
If I were to suggest books I would never tell anyone to start with the first two novels as they were not his best although some love them. Im kind of one of those people that advocates reading by the characters, you have the City Watch series, the Witches series, Death series, Rincewind the Wizzard series, Moist von Lipwig series of books, and the younger audience Tiffany Aching series. There are several other novel standalone novels as well. If I were to pick I would go with the City Watch or Death Series first or the standalone book Small Gods. The first City Watch book is Guards! Guards! and the first Death book is Mort.
I have seen Small Gods recommended before, so that's probably a good place to start. I decided just to read them in order, and didn't have a problem with the first two, although I agree they're not the strongest. I'm only up to the 7th (Pyramids), so Mort is the only one I've read that you mentioned. I loved it, but I've really enjoyed the Witches books, too.
Nightwatch (my favorite), Guards! Guards!, Going Postal, Small Gods, and Hogfather are my fave five.