I really like the story in Halloween 3, and I agree it would have been better received if it wouldn't have been marketed in the series. I've read that originally Michael Myers was only supposed to be the focus of the first movie, and the series was supposed to be a series of different Halloween stories. Michael Myers became the focus and the focus shifted. I hated H3 the first time I saw it because I was expecting a Myers movie. As I rewatched it later, it grew on me. Halloween is still my favorite horror movie, and I like most of the sequels as well.
I always watch Ernest scared stupid in October, but I'm not sure if it counts as underrated