This is Mike of the Mike and Lor husband wife combo! I'm a huge TP fan and love all Discworld. I've selected several for my wife and she's loved them all, except Reaper Man oddly enough.
I would recommend Guards! Guards! to start the City Watch series as the Watch books are the most consistently great. I started my wife with Maskerade which she enjoyed. My personal favourite is Small Gods which is a great stand alone novel. The Hogfather and Thief of Time are fantastic all rounders and I would suggest Lords and Ladies as the best Witches novel. Or Carpe Jugulum.
Essential love them all. You can do no wrong with starting in The Colour Of Magic and reading them in order. It's true in the first three (Colour Of Magic, Light Fantastic, Equal Rites) he is finding his voice in Discworld but that doesn't mean they aren't great, just a little inconsistent withered books when he settles.
Ahh I want to read them all again now. On Game Of Thrones right now. Bit different.
Enjoy!