Rather than watch just a short list, how about putting in the time as a general Star Trek fan and watch the majority of episodes?
If you want to avoid the disappointments, I suggest you skip
- The Alternative Factor
- And the Children Shall Lead
- Plato's Stepchildren
- Whom Gods Destroy
- The Way to Eden
- The Cloud Minders
- The Savage Curtain
You'll miss a lot of things that we're nostalgic about that aren't really that great, and this way you'll see all the really good episodes.
The "Must see"'s mention was just the spur I needed to watch the whole series though I would like to skip the disappointing episodes. If anyone objects to this list or wants to add to it, I'll take your word for it.