Most underrated Disney film?

In a sense, TGMD is what saved Disney animation and allowed the Disney Reniassance to eventually take place. After the critical and financial flop of The Black Cauldron, many thought that it would spell doom for animation in general and Disney actually considered shutting down their animation studio. While The Great Mouse Detective wasn't a major hit, it was successful enough to convince the studio that animation was still a marketable enterprise. And the rest is history.