Star Trek's most popular animated episode, some might guess, is a sequel to a well beloved episode of the live-action series.
Unfortunately, episodes are no longer available in full form. Here, then, is the trailer for "More Tribbles, More Troubles":
Out of Filmation's freshman class of 1973, Star Trek, unsurprisingly, was the most successful, but it wasn't the only adaptation of a TV series the studio produced that year. It just happened that Lassie's Rescue Rangers (ABC) & My Favorite Martians (CBS) flopped, and I think it might be because either one or both were slotted opposite Trek if my memory serves me correctly.