I really like the Guards series, they start with Guards, Guards and I think you'll enjoy Lady Ramkin. If you want a standalone my vote is for Going Postal - though actually the follow-on to that is being published in October, to my great joy because I thought it had the best romance I've ever seen from Pratchett. (I did like the choice of Ysabell, but we see little of the actual romance.)

Mely is right about Colour of Magic. Just keep going from Mort is my advive - I enjoy the witches books also, they start with Wyrd Sisters, which is very much Pratchett Does Macbeth and fun. Try it out! Try them all out. It's lovely to have an author as consistently good as Pratchett turning out regular books, it makes me happy.