On the air from 1966-1969, TOS is the one, the only, the first Star Trek to ever exist. It got the ball rolling, and no matter what your opinion (ideal, perfect, sullied by all that came after OR kitchy prototype that made possible all the good stuff later) it goes unrecognized by no one in the Trek universe.
Pitched to the studio as “Wagon Train to the stars”, TOS was Gene Roddenberry’s vision of an idyllic future where peace and harmony reigned on Earth and Humans sailed the stars solving problems and having adventures. The key feature of TOS is the Triumvirate of Captain Kirk, First Officer Spock and Doctor McCoy representing Ethos, Logos and Pathos. Their interplay completely overpowered every other character and drove the central thesis of nearly every episode and most of the movies. TOS explores how the different aspects of the Human experience can be useful in solving problems. The theme of TOS is a little difficult to nail down, but most episodes revolve around open-mindedness, teamwork and ingenuity saving the day. It’s a fun show that completely bombed the first time out and eventually gained a following and paved the way for everything that was to come (after almost destroying everything that was to come with an ABYSMAL first movie).