My two bits no one asked for on ranking Discworld series
>City Watch : absolutely the most consistently good one up until Snuff, hard carried by Vimes but watching the cast go from "four guys with trucheons trying to not get beat up too much" to the greatest legal organisation of the setting is a big draw. Also good whenever the cast show up in other books.
>Witches : almost tied with City Watch, Weatherwav and Ogg are the best and have basically NO bad POV. But sadly the third witch (whether it's Magrat or Agnes) is always horrifically, horrifically dull.
>Moist von Lipwig : really good series (at least the first two books) that sadly had to be shorter than what Pratchett intended (because he died). Unconventional protagonist and coming into the Discworld when the setting was already fully fleshed out helped a lot
>Tiffany Aching : Witches series but with a younger protagonist. Underrated as fuck but nothing much to say about them, Pratchett probably wrote these books because he was sick of Ankh-Morpork
>Death : overrated as fuck. Death is usually fine (probably why Reaper Man is the best of his novel and probably in the top 5 Discworld overall), but holy hell everyone else he has to share the splotlight with is so dull, ESPECIALLY Susan.
>Wizards : the worst series BY FAR, Rincewind isn't funny or interesting and Ridcully can't carry the load alone. It's so boring that most of the recurring characters don't even have names.

Concerning the various oneshots novels, it goes :
Small Gods > Thief of Time (technically Death, but the best part is the standalone characters) > The Truth > Monstrous Regiment > Pyramids > Moving Picture

Small Gods being, of course, easily above most others Discworld novels.

Never read the Amazing Maurice, never got the impression I missed much.