Guards! Guards! is a great starting point, both on it's own merits and as it introduces the Watch series which is terrific. It was also the author's favourite (or one of his favourites).

Be aware that if you start at the beginning with The Colour of Magic the very early books are a quite different style to once he'd got into his stride, so unless you're absolutely determined to read them all I wouldn't start there.

Edit: Btw, Guards! Guards! is early enough in the series that it's still recognisably 'early' Pratchett, so if you're interested in seeing his style develop it's a great place to start for that reason too.