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.
As others have stated, it's all a matter of personal taste. My comments on the list above.
The Alternative Factor - A ridiculously over maligned episode. Personally, I love it - DO NOT skip.
And The Children Shall Lead - Not great by any means (well short of greatness in actuality), but not truly horrible either. Worth a watch at least once.
Plato's Stepchildren - Pretty good actually, although some of the "antics" might either have you thinking WTF, howling with laughter or both!
Whom Gods Destroy - I like this one quite a lot. Lord Garth is awesome!
The Way To Eden - This is my least favourtie TOS entry. If you must skip epsiodes I guess you won't really be missing anything here.
The Savage Curtain - One of my favourites from Season 3 - DO NOT skip.