Pyramids would probably be my one book as I am a child at heart and would love to have the adventures of the assassins (without inhuming anyone of course!) Plus I love history so the pyramids and Dios won me over straightaway.
Nightwatch for me was the best written and most engaging. Sam Vimes could easily a real person in Roundworld and due to that fact it is so easy to relate to him.
I tend to read series together - witches, city watch, death etc. I have this tendency that if I fancy reading a book, say Witches Abroad, I have to go back and read Equal Rites etc. before I read WA but then of course I just can't stop and have to read the rest of the series.
On the flip side of the coin I have to say that my least favourite character is Rincewind and I don't tend to read his books very often. I've never been one to like the anti-hero who survives through pure chance as I can't relate Rincewind as I am the type of person to plan everything.
Love all PTerry books in general - just keep them coming!