Hmm. The way the thread is titled, I assumed it meant episodes. I remember (from back in 5th grade, over 40 years ago) seeing the end of "A Taste of Armageddon," the beginning of "Space Seed," and I'm pretty sure "Devil in the Dark" was the first one I saw all the way through. And I would be hard-pressed to decide on a single favorite episode, even just TOS, much less across all series.
But a favorite series, other than the first I encountered (TOS)? TNG, hands-down. Probably the purest expression of Roddenberry's vision.
But a favorite series, other than the first I encountered (TOS)? TNG, hands-down. Probably the purest expression of Roddenberry's vision.