The worst is "The Alternative Factor." Perhaps someone has a great explanation as to why it's a terrific, catchy title. I am prepared to be educated.
Not-so-great ones include the phone-in, whoops-we-need-a-title jobs such as "Assignment: Earth," "Operation: Annihilate," and "The Enterprise Incident."
There are many, many good ones. For me, the best TV episode titles are ones that make you think, evoking imagery and feelings that really bring home the themes of the show while conveying ideas that last well beyond its running time. "City on the Edge of Forever" is a fantastic, fantastic title and may be the best even though I don't find the episode itself to be among my personal ten best, let alone the best of TOS. The Shakespearean titles are good, particularly "Dagger of the Mind" and "All Our Yesterdays," with the same play as the source, no less. "Dagger of the Mind" gets extra points because it's not an exact quote. Then there are those that sound Shakespearean, but aren't, like "The Savage Curtain."
Upon reflection, the best, for me, is probably "The Devil in the Dark."