Okay, since most of you have taken my choice for Edith Keeler's death for sheer emotional dramatic impact, I'm gonna go with Kirk's most Bad-Ass moment, and narrowest escape...
"Now! Energize and detonate!"
When I was a kid, "Obsession" bored me stiff (and the cloud creature effect looked really lame), but that last few minutes, where Spock & Scotty are desperately trying to retrieve Kirk's & Garrovick's scattered molecules while the ship is being buffeted by shock waves from a planet that has just had its atmosphere stripped away... well, it was quite the nail-biter.
Now, watching as an adult, this is one of my favorite episodes, largely due to the evolution of the question, "Is this a creature
, or just a random gas cloud?" to "Is this an intelligent
creature?", then finally, "Does this intelligent creature pose a threat to the rest of the galaxy?"
Like "Devil in the Dark", this was one of the best examples of the "seek out new life" tagline of the show.