Hunchback definitely fits the category of underrated for sure. I picked that one. I enjoyed Hercules and Mulan too, though I wouldn't consider the latter underrated, necessarily. The Emperor's New Groove is definitely underrated too. Especially since it was released after the Disney Renaissance.
Hunchback is definitely underrated. I don't know who in their right mind thinks that Mulan is underrated. Like, are they serious? Mulan is a Disney Princess. All Disney Princesses are on the radar and certainly not underrated at all.
Out of those, it'd be the Hunchback of Notre Dame because 'God Help The Outcasts' and 'Hellfire' are two of the most striking Disney songs I've ever heard. However my ultimate favourite underrated Disney film has to be Atlantis.