I'll probably suggest Maskerade, or Wyrd Sisters. I think these two are good starters and would totally hook up the new reader.
For the Death series, I recommend Mort. I haven't read anything about Rincewind, and I'm not a big fan of the guards either.
Hm, this got me thinking. How did I start with the discworld. Well, it was with a girlfriend of mine, we were at the book market, and we saw these 10/15 books with these SUPERB covers/drawings. We totally loved them. The next day we both came to buy one of those books... And we both wanted Witches Abroad. And it was the last one.
We fought and fought and fought and she won. Ahum, I mean, I have always been a gentleman, so I let her have it. 8) I ended up buying Equal Rites...