And I've recently heard that Stegosaurus was the only stegosaur
to lack shoulder spines, not sure if this is true. If it is, it
would be the most derived of them all.
This would not tell us anything about its phylogenetic position, however.
The term "derived" should be restricted to character states. For whole
organisms it only makes sense, if at all, when it's applied to the average
of a large number of character states.