Star Trek: Deep Space Nine
Spacerguy has explained in his participation of the challenge this year how Worf ended up in The Next Generation. I mention this because originally Gene Roddenberry didn't want Klingons, or any other familiar aliens, in Picard's adventures. Instead, it would be all-new ones, like the Ferengi.
Now, the Ferengi as originally depicted in Next Generation were...not quite worthy successors to the Klingons. There was the episode ("The Battle") where Picard's backstory begins turning things around a little, but it wasn't until Deep Space Nine that Ferengi could truly be taken seriously. Still, "Ferengi episode" is a common term among even this show's fans, and it's not uttered with affection. In fact, the episode that's generally regarded as the show's worst, "Profit and Lace," is of course a "Ferengi episode."
Perhaps tellingly, every time Deep Space Nine visited the Mirror Universe (the alternate reality first introduced in the original series episode "Mirror, Mirror"), a Ferengi is killed off.
All that considered, especially in Deep Space Nine, I personally loved the Ferengi. Bartender Quark, ever the scheming pragmatist and therefore thorn in the side of Odo (chief of security, shape-shifter, eventual love interest of Kira Nerys), alone did an excellent job of redeeming them. Eventually his brother Rom and nephew Nog became worthy companions in this crusade. "Little Green Men" is the episode where they all undeniably shine together.
It's a time travel episode. As the title implies, it's one of science fiction's many explanations for Roswell, New Mexico, and its long association with UFO conspiracy lore. Nog has become the first Ferengi admitted to Starfleet Academy, but of course this doesn't stop Quark from attempting to make a side profit as the family journeys to Earth, which backfires so spectacularly they end up thrown into the past.
Watching as Army officers attempt to understand these strange aliens, before a bobby pin is used to repair Universal Translators hilariously hidden inside the outsize ears of our unusual heroes, it's pure comedy gold.
To say nothing of a more subtle in-joke that sees Nog digging through Earth history and coming across the profile of Gabriel Bell, who looks suspiciously like Captain Sisko. This is because Sisko became Bell in the classic two-part "Past Tense" the previous season. (You could do a lot worse to introduce yourself to the whole series, if you've never seen Deep Space Nine, with a viewing of both "Little Green Men" and "Past Tense." And then throw in "Trials and Tribble-ations" for good measure. And then "The Visitor," one of the best episodes of the whole franchise.)
All of which is to say, this is definitely an experience you need to see to truly grasp how awesome it is. It may even have you liking Ferengi, and then you will have a totally different interpretation when you hear the term, "Ferengi episode," because there are plenty more good ones to discover.