Star Trek The Next Generation: BEST EPISODE EVER, Best Worf Episode, Best Data Episode, Etc.
For me, the one episode I always go back to is Emergence, or The Chase. Both challenge our definition of life, how we define consciousness, and how those perceptions affect self identity. Christ that sounded gasbaggy!!!! I also love how those episodes touch on those themes without using one of the characters as foundation for the episode. Dont get me wrong i love how Data and Worf were often used as foils for how we view humanity, but those were specifc to the arcs of those characters. Emergence and The Chase challeenged the viewer to imagine life on a larger scale. Ugh more gasbagginess!!!