The two-parter I think is underrated is Gambit. I just don't understand why it's viewed as weak. I thought it was a fun show (watching Picard blending in as one of the crew, hating Riker and stuff was pretty funny).
I won't speak for anyone else, but for me the bad part about TNG: Gambit
was the following.
In TOS: The Savage Curtain,
Surak was described as the savior of Vulcan, because he led the development of their philosophy of peace and reason which in the end won out over violence. From http://www.chakoteya.net/StarTrek/77.htm
SURAK: In my time on Vulcan, we also faced these same alternatives. We'd suffered devastating wars which nearly destroyed our planet. Another was about to begin. We were torn. But out of our suffering some of us found the discipline to act. We sent emissaries to our opponents to propose peace. The first were killed, but others followed. Ultimately we achieved peace, which has lasted since then.
Now, before you say that that wasn't the real Surak, from the same episode:
It is certainly true that the image of Surak was true to how Spock understood Surak to be.
KIRK: Thank you, Mister Chekov. Mister Spock, any explanation?
SPOCK: Conjecture, Captain, rather than explanation.
SPOCK: It would seem that we were held in the power of creatures able to control matter and to rearrange molecules in whatever fashion was desired. So they were able to create images of Surak and Lincoln after scanning our minds and using their fellow creatures as source matter.
KIRK: They seemed so real. And to me, especially Mister Lincoln. I feel I actually met Lincoln.
SPOCK: Yes, and Surak. Perhaps in a sense they were real, Captain. Since they were created out of our own thoughts, how could they be anything but what we expected them to be?
I'll take Spock's opinion as about as authoritative it gets in-universe in Star Trek. Therefore, the image of Surak's description of Vulcan history was accurate.
So, what's wrong with TNG: Gambit
is that it betrayed and cheapened this view of Vulcan history. Instead of Vulcans taming their animal passions with reason, the need to adopt logic was reduced to rendering inert a random artifact-of-the-week that mattered neither before nor since in on-screen canon. It was embarrassing.