It's been over fifty years since the original Star Trek television show first went on the air in 1966. Not only has the program been lauded for its impeccable forward thinking, through either intelligent progressive commentary, or its sci-fi fascination with future tech, but it was also the catalyst for what was to become a pop culture phenomenon.
Since TOS concluded in 1969, Star Trek has spawned five more successful television shows (including an animated one), with a sixth quickly approaching its debut. On top of that, there have also been thirteen feature films carrying the Star Trek name, with a fourteenth currently in the works. That's pretty much the perfect testament to the importance of the show.
The ground-breaking show was fairly exceptional in its own right, but some episodes stand as important monuments beyond their entertainment factor.
That comes down to a number of factors. The episode could have presented damning social or political commentary that can still be seen as relevant today. It could have established elements that would become integral to this giant of entertainment. It could have represented a landmark achievement for the show or it could have simply been so good that it is required viewing for fans. Or it could have been a combination of any number of those things.