TOP THREE

Top three descriptions of Star Trek (The Original Series) season one episodes (from Wikipedia):

1. Dagger Of The Mind: At Tantalus V, a rehabilitation colony for the criminally insane, the inmates have taken over the asylum using a neural neutralizer, which is used to remove (or implant) memories from a subject's mind. One colony administrator, Simon van Gelder, escapes to the Enterprise, leading Captain Kirk to investigate the colony. While on the surface, Kirk is brainwashed and taken prisoner, but is helped by Dr. Helen Noel, a colleague from the Enterprise who joined him on the planet. Spock performs a mind-meld with van Gelder to counteract the effects of the neutralizer, healing his mind and allowing van Gelder to take over the colony after Kirk and Noel's rescue.^[36]

2. A Taste of Armageddon: On Eminiar VII, the Enterprise finds a civilization at war with its planetary neighbor. Unable to discern any signs of battle from orbit, Captain Kirk leads a landing party to the surface where he discovers the entire war is fought by computer. Even though the war is simulated, citizens who are listed as virtual casualties still report to termination booths to be killed for real. After the Enterprise is destroyed in an attack simulation, Kirk must fight to keep his crew from death.^[36]

3. The City on the Edge of Forever: After accidentally overdosing on a powerful stimulant, Dr. McCoy acts erratically and disappears through the Guardian of Forever, a newly-discovered time portal on a remote planet. Kirk and Spock follow after learning that McCoy somehow changed history. Arriving in the 1930s, the duo meet Edith Keeler, a New York social worker who gives them a place to stay. As the days pass, and McCoy is nowhere to be seen, Kirk finds himself falling in love with Keeler... but Spock discovers that Keeler must die to restore the timeline.^[36]