With respect to number6's list above:
Absolutely, "City on the Edge of Forever" and "Cause and Effect" are great episodes, and I would count separately (i.e., as a meritorious comedy) ST: IV. But is anything else on this list really that enduring? "Time's Arrow" was as contrived as could be, and all that really sticks with me is the fairly convincing Mark Twain. And although "Tomorrow Is Yesterday" had many fine moments, the resolution of the story ("replacing" the Earth guys into their lives such that they didn't remember) never made much sense.