I think Meet the Robinsons is one of the most underrated Disney movies. It has such a good message to it, which is to keep moving forward no matter what or who tries to stop you. And the fact that they used an actual quote from Walt Disney as a theme for the movie always makes my day.
Another really underrated but more recent of the Disney movies I think is Saving Mr. Banks. I never hear people talking about it when it's actually one of my favorite Disney movies. It sheds some light on the process it took for Disney to get the rights to PL Travers book, Mary Poppins. Tom Hanks portrayed a wonderful Walt, practically perfect if you ask me. I loved seeing the process of them creating some of the Mary Poppins scenes as that, too, is one of my favorites. It may have been sad, but that doesn't mean it should be overlooked.