I can't rank them because it changes from moment to moment, but I agree that Equal Rites is weak. Part of it may be that it was not originally a Discworld story, but was adapted to fit. The places where the copyeditor missed changes that needed to be made show that. Granny supposedly taught Esk for three years, but she begins when Esk is eight and Esk is still eight years old when she arrives in UU. Esk also originally had only six brothers and was the seventh child. There is at least one scene where all the brothers are supposed to be mentioned, but there are only six names, and there is no indication of the seventh brother. Esk seems to have been originally eleven (the usual age for young British protagonists, probably because of the "eleven-plus" examination that used to determine their future education) and then made younger. This idea supports her naivete but it makes her wide range of training unbelievable to the point that the reader suspects she is lying to the boat people she travels with. The true main character is Granny, who is a new creation, nothing like what she becomes, but she is there at the birth and learns more during the trip to Ankh-Morpork than she expected to. Esk, as a minor character (pun built into the story), provides an external point of view for us and also gives Granny a way to be involved with the wizards as an outside agent. Granny is there at the end of the book in the last scene, being treated as at least an equal. It's a reasonably good first adventure for Granny.