I've heard his compositions outside of Michael Jackson albums and there are some hallmarks that should rate him in an elite class of songwriter (even if the popularity/appeal of those songs are very much confined to one specific era).

I do think he does get excessive credit for his contributions to the Thriller and Off The Wall albums, as a lot of the key songs of both are Michael Jackson contributions (some even reach that "credit" backwards on albums neither Quincy and Rod had anything to do with), however.

Working with Michael Jackson only raised their stock IMO... he was the rare kind of vocalist that could meet the demands of some of those songs.