I read them originally in the order they came out.
I've been reading them to my son for about 4 years (We keep saying stop, but it's still fun)
After we finished with Amazing Maurice and the Tiffany books, I did a few Witch books before breaking out into some Death, some Vimes and then Rincewind books.
Carpe Jugulum needs to have Small Gods as a pre-cursor, else Oats make less sense.
Both Carpe and The Fifth Elephant help introduce Igors and make the Thief of Time more sensible.
You need to have Gaspode explained in Moving Pictures before he becomes realistic in 5th Elephant or The Truth.