#15: The Empath
In case you forgot: The Power Trio are trapped in some weird lab with aliens and a woman who can't talk.
This episode gets a lot of flak, but I really like it. I'm not afraid to admit it! The twist that the aliens are running an experiment to see if the mute woman's race is worth saving is some nice piece of philosophical sci-fi. The way that episode builds to that reveal is nice—for a while it seems like the aliens just might be sadists without any kind of purpose—as is Kirk's speech to them at the end. Hero points also go to McCoy for refusing to let the empath heal him, preferring to die himself than to put her in danger. He takes that Hippocratic oath pretty seriously.
A race of beings who communicate through pure feelings and sensations without the use of any intermediaries like language is also a cool idea (that doesn't get explored at all, but still). What would they do for fun? Would they have anything resembling art or music? Would their entertainment be something like "emotional concerts" put on by beings of unusually strong feelings?
That said, I can see why this episode's not a beloved fan favorite. "Gem"? Really? Aside from the rather lackluster set, the aliens are also kind of needless jerks at the end. Gem's obviously passed the test as they initially set it to be: she was willing to sacrifice herself to save someone else, and it was pretty much McCoy's doing that prevented her from sealing the deal, yet they still insist that she failed? But it gives Kirk a chance to make a nice speech, so, not too bad.