I'd say "The Menagerie" is probably the most overrated episode. Seeing the first pilot adds a nice bit of back story, and Spock's decision to violate regulations in Part I is a strong, dramatic hook, but Part II is a dud where the characters spend most of the episode watching TV punctuated by contrived cliffhangers at the act breaks which are all too easily resolved.
And yet, it's a Hugo winner. Season one had a number of episodes which were much, much stronger.
I think it was primarily the pilot component that earned the award, along with the 'layered' effect of seeing two stories at one time. And considering that the whole purpose of the episode was to frame the pilot, it's not surprising that a good part of the episode consisted in showing it.