The Watch series is my favourite, I love Sam Vimes as a character. The witches run a close second. The first of either of those would be a good start.
The Colour of Magic/ The Light Fantastic do a lot of sending up of established fantasy authors, so if you aren't into the genre they're a bit hit and miss.
Death in Reaper Man is beautifully done, and Maskerade is a hilarious send-up of Phantom of the Opera. TP hasn't written any really duff books - a dodgy TP is still miles better than nearly everything else.
I'm reading them to the DDs right now - we've done Mort and Reaper Man, the entire Witches series, all of Tiffany Aching and are now on the Watch series about to start Fifth Elephant (which is a bit dark, but very good).
I envy you.