My father was a TOS Trekkie and since I was a kid, we've gathered in front of the TV to watch TNG, DS9, Voyager, and all of the movies but I never got around to watch TOS all the way through. I've heard mixed feelings toward the series and instead of watching every episode, I was wondering which are the best/must see episodes of TOS. Watching every episode of the following series and movies, I get the story but I'm looking for those "oh, that's how that started" or "that's where he is from" episodes.
Their are top 10 threads but I'm also interested in continuation stories followed up in the later series.
A few suggestions, if what you're looking for is sort of how things got started.
The second pilot, Where No Man Has Gone Before. Kirk is the captain, Spock is there, but the rest of the crew isn't there yet, and it's interesting to see how the show changed from WNMHGB.
The Menagerie, a two part episode which includes footage from the first pilot. It gives Star Trek the feel of an actual world with a history and Spock a backstory.
Balance of Terror - where the the Romulans appear.
Errand of Mercy - where the Klingons appear.
Amok Time - which has Vulcan history and, um, interesting mating rituals.