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This is why I think thread like this need more first post defining of terms. To me, under/over-rated suggest what those that know a thing think of the thing. If more people should know about the thing, that would be a thread about what would be more popular. Captain Power and the Soldiers of the Future is a prime example -- nearly everyone who still knows about the thing seem to agree that it's wildly better than it need to (or had any right to*) be. Based on my interpretation, this falls into correctly rated. But if not-known-by-enough**-people falls into underrated, than it decidedly is underrated.
*based on being an 80s toy-commercial tv show, not based on the talent behind it
**as many as ought to know about it
This is another 'what are the definitions?' situation. To my mind, Babylon 5 and Deep Space 9 are both over-rated by geek culture because -- well, how could they not be? Nothing that is merely great television could live up to the online geek consensus surrounding these shows. Most non-universal cancer cures or lesser forms of world peace would be over-rated if they had the same general perception as these two shows have with online geek culture*.
*western audience of young boomer to young millennials who would have been watching tv in the 90s that... etc. etc. etc. (whatever the actual demographics are, you know what I'm getting at).
Anyways, that's a bunch of theorizing about the boundaries of the question. Trying to match how others are using it (what isn't good that lots of people say is; what is good that not enough people say is), I will nominate:
Overrated: the sci fi work of Larry Niven. Not as popular as he was in the 70s and 80s, but still pretty universally acclaimed when brought up*. Here my main complaint is repetition. I enjoyed the first of his works I read (The Mote in God's Eye), but each subsequent book I read I saw the same characters (with different names) going through the same dynamics, setting up the same man-of-action vs women/intellectuals within group conflict surrounded by a different interesting sci-fi set piece. *keeping politics and personal foibles out of the discussion.
Underrated: This breaks my personal interpretation of the rules, since most people who know of them do like them*. However, I think The Chronicles of Prydain are really solid young adult literature that deserve both more praise and attention, and recognition as one of the first and more influential post-Tolkien fantasy series that helped define the field in the 60s. There's no denying that Alexander saw the Hobbit/LotR come out and said, 'this is the template I should follow,' but he already had the seed of his story from being stationed in Wales during WWII, and that gives his tale a uniqueness that other immediate-Tolkien-followers lacked. They are interesting novels worthy of sharing with your kids.
*minus those who only know the Disney movie and dislike it.