WaLGMO? was the first episode I ever saw. Unfortunately, I came into it around halfway, and the fake cave set gave me a bad impression, among other things. I liked Ted Cassidy's work in it though.
For DotM, it's performances. One highlight for me is Kirk's breakdown in the chair at the act break. The first on-screen mind meld is mesmerizing, including its mandatory acknowledgment to standards & practices that Spock was not performing hypnosis. Nimoy and Woodward worked the scene wonderfully, a vivid contrast to VanGelder's earlier insanity. And James Gregory was deliciously evil.