Encounter at Farpoint (Star Trek: The Next Generation, Season 1, Episode 1)
This is the very first episode of Star Trek: The Next Generation -- affectionately called TNG -- and the first one where we meet Q, a recurring antagonist. All of the Q episodes are really, really great. In this one, he insists on putting all of humanity on trial. This episode also marks the first appeerance of the man-skant.
Watch Star Trek: The Next Generation on CBS All-Access.