That is my favorite episode too. Its call "This Side of Paradise" or something like that. I always though the two best sequences in terms of acting we in that episode. Where Spock is hit with the spores and has a negative reaction at first, and Captain Kirk on the bridge alone speaking his entry in the log.
Good stuff ...
Yeah the acting in that is the best, Kirk by himself getting mad, "no no no no, I- won`t - go - down-" then when he has to get Spock mad, "You pointy eared half breed! Your side show freak, You dog boy!" ha ha! That`s what made that show, just a lot of screen presence and strong characters. I remember when I was a kid watching that, I thought they were real people, then they had Star Trek the next Generation which I thought was a joke. "Star Trek the politically correct generation" I thought who the hell acts like that in real life?