I'm going to stand with Kater on this one. Hunchback has one of the best soundtracks Menken put together. "Hellfire" probably isn't my favorite from the movie, but I've heard other people call "Hellfire" their favorite, too, so Kater's not alone. However, it would likely never appear on a mainstream list because it's part of a second tier of Disney animated musicals that people don't remember as fondly; others on that list might be The Princess and the Frog and, to a lesser degree but still sadly true, Hercules (a favorite of mine).
Hunchback's soundtrack also is less "musical pop" and more straight-up musical with a really dark tone. The songs don't seem to be written to be catchy or fun. Its most upbeat (and positive) song is still nowhere near songs like The Little Mermaid's "Kiss the Girl" and "Under the Sea." I think that's another reason it doesn't have the popularity other soundtracks have.
Hunchback's animation doesn't look very good in my eyes, perhaps due to the art style they were trying to emulate, so I get why it hasn't survived as well. I mean, compare it against the early pastel backgrounds from Beauty and the Beast and it quickly loses. It's also a dark story (based on an even darker one) where the hero is a deformed man whose life never really noticeably improves, rather than the story of a beautiful girl whose life goes from average to perfect. But that soundtrack holds up very well.
As for my own favorite...I can't make a choice. Depends on the day, and I need a list of at least 20.