There so many great episodes I'll just list one from each season (know that tomorrow my list will be completely different:) From Season 1 Balance of Terror, space battle at its finest! From Season 2 Amok Time, Vulcans can be very complicated. From Season 3 The Tholian Web, weird as all hell, but very cool! Honorable mention if you are a Shatner fan "The Enemy Within" Billy did a great job pulling this off. Honorable mention if you are a Nimoy fan "The Enterprise Incident" romulan CC was a hottie!