STAR TREK TURNS 50: Here are the 13 best 'Star Trek' original episodes, ranked

"Star Trek" is arguably the greatest science-fiction television series of all time - if for no other reason than its lasting influence on the genre.

The show addressed questions relating to human rights, social issues such as racism and nationalism, the Cold War, the United States' role as a superpower, and the possibility of a global war. Meanwhile, it (usually) promoted tolerance and hope.

Plus, the cast included Nichelle Nichols as Lt. Uhura, a breakthrough role for black women in American TV, and Walter Koenig as Chekhov, a Russian - both of which definitely sent a message in the 1960s.

And so, in honor of the 50th anniversary of the first episode's airing this Thursday, we ranked the best 13 episodes from the original series. Whether you love the show or have never watched, it's time to dive in (all episodes are streaming on Netflix).