The Prime Directive is a pretty cool idea. Star Trek wasn't always clear on what it meant, but the general idea is that the Federation and its representative must not interfere with the development of alien civilizations. It's a cool enough idea that I've heard crackpots who claim aliens have visited Earth haven't identified themselves because they've got a Prime Directive of their own.
"The Cloud Minders," an episode from the dreaded third season of the original series.
I remember seeing this one as a kid, and being impressed with the basic concept. Kids don't think too much about the organization of society, so any discussion of it on a TV show could be mind-bending. The episode is about a culture (actually, a planet, but Trek tends to believe in one culture per planet) where the leaders--who devote themselves to art and higher things--live in a city on a cloud, while most toil in the caves below, digging up a valuable substance known as zenite. In the unfortunate Star Trek tradition of names based in English, the cloud-city is called Stratos and the cave dwellers are Troglytes. The symbolic clash here may be a bit obvious, but it's light years more subtle than "Let That Be Your Last Battlefield."
It's discovered that Zenite gas effects the troglytes, making them stupid and combative (so the elite have a point). But the effect is only temporary, and McCoy comes up with a gas mask that will protect them. Kirk wants to give all the Troglytes these masks, aka, shred the Prime Directive.
Before he leaves, Kirk promises, now that he's turned their society upside-down, that the Federation Bureau of Industrialization will come in and help them reorganize everything. So I guess the Federation is used to this sort of thing.
Actually, the show's metaphor works amazingly well for the modern struggle we see in society. Many leaders believe (or at least used to believe) that they must ensure basic freedoms for their citizens--that the state must not interfere with personal beliefs and activities, or at least intrude as little as possible. Meanwhile, others feel that the citizenry is their project, to shape and mold into a better society, and if this means preventing them from saying stupid things or thinking stupid thoughts, it's well worth it. So well done, Star Trek. The struggle continues. Free the Troglytes.
PS A two-volume oral history of everything Star Trek is now available. The combined length is over 1400 pages. Luckily, as the first book covers 1966 to 1991, that will take care of just about everything I'm interested in. (As I write this I see the first volume has three times as many customer reviews as the second, so I guess I'm not alone.)