#1: Amok Time
Here we are, everyone. My favorite episode and the end of my massive Star Trek: The Original Series retrospective. Other blogs have done it before, and better, but everyone should have a go at it.
In case you forgot: The infamous pon farr episode. Spock has to get back to Vulcan to mate. His chosen bride doesn't seem too keen on getting with him, which results in Kirk and Spock battling to the death. Or the fake death, in this case.
"Amok Time" pretty much represents the ideal balance of a good drama (best friends forced to fight to the death) and solid science-fiction ideas (the Vulcans and pon farr). Nimoy's performance is without equal; honestly, it's probably the best over the course of the entire series. Proof that less is more: Spock's smile and the way he calls out Kirk's name when he sees him again in sick bay is everything that ever need be said about their friendship. As far as I'm concerned, it's the best moment in TOS.
Besides the drama and sci-fi balance, "Amok Time" also has the ideal balance between our Power Trio. Everyone has a part to play, and it's all equally important: Spock is the unwitting antagonist (and victim); once on Vulcan and Spock's safety ensured, Kirk is the endangered one; McCoy is the quick-thinking hero that saves Kirk's life and Spock's reputation. That's also a rare thing in TOS.
It's the golden mean and happy mediums all over this episode. I love it. I never get tired of it.