Star Trek, TOS: The Naked Time
One of the most interesting episodes, IMO, because it brings together the revealing of our "true selves" and the concept of time travel--which foreshadows several other ST:TOS vehicles. I like that early on, Roddenberry was determined to exhibit serious concepts and explore them.
I especially like the interaction between Spock and Kirk when Spock is crying over his mother and then Kirk seems to respond with rage and then he realizes that he has been infected as well.
Nurse Chapel reveals her love for Spock and the more I see this episode, the more ambiguous Spock's response is. I also like Uhura's role in this episode.
O'Reilly is still annoying, though.