Lords and Ladies was my favourite for a long time.
I was going to post that I wouldn't necessarily recommend it for someone interested in sampling the series, but now that I think about it, it is one of the very best ones, showcasing his style very well. It's just that it's sorta a sequel to Witches Abroad.
Pyramids is widely considered a good introduction to the series. It's typical of many of the later books without relying on characters that have been around before. It also has scenes like the test in the Assassin's Guild. Small Gods is a good standalone book if you think the person may not read any others anyway.
The Truth is also possibly a good starting point, except that so much of what makes it good is a) what someone tries *again* (i.e. knock off Vetinari) and b) what is now being allowed from that book forward (i.e. movable printing).
Wade.
