'Underrated' and 'Overrated' aren't words I feel you can apply to anime, since it's mostly sidelined by the mainstream and there aren't
any credible figures out there to rate this stuff and have it stick. It's more a case of 'I'm sick of people talking about X' or 'I wish we'd talk about X more positively', and half the hate any title gets is simply about how popular a subject it is, rather than whether it's actually bad or not.
Case in point; the idea that any Gundam series is less or more rated than the other just seems trivial to me, since:
1) Anime is a niche interest. Mecha Anime is a niche of a niche interest. I'd be surprised if the 'rating' is being done by a population in triple digits.
2) Having watched nearly all of it, I conclude that there is no 'best Gundam series', just a 'least worse Gundam series' anyway
The only thing I feel has been overrated in Anime is
Akira. It is credited with creating the western interest in 'Anime', but only in the widest of senses. It created a type of Anime fandom that has little if nothing to do with what people are interested in today, fitting comfortably into the existing comic-book / super-hero genres. It's a good, brainless action movie with some stunning art, and a memorable soundtrack, trying to drag itself of its otherwise lacking notoriety with some typically uninteresting and poorly written BS about life, the nature of energy and the rest of human existence. The characters suck, and it's not nearly as bad as the
Ghost in the Shell movie on every count, excepting that the only thing GitS ever created in anime fandom was wrist-strain.