From my viewing experience, I'd have to say that Star Trek The Animated Series strikes me as the best cartoon series from the 1970s. I'm not saying this as a Star Trek fan since the cartoon got me interested in regular viewings of the series (Mostly the original).
While I do enjoy some of the typical 70s cartoons, Star Trek TAS makes for a nice escape from the usual conventions (teen sidekicks and cute animals).
While I can certainly see why Gene Roddenberry didn't think too highly of it compared to the other shows at the time, I think he didn't give it enough credit.
For one, some reviewers at the time I believe made statements about how "the show is too good for Saturday Morning" says a lot about the cartoon.
Second, a good number of adaptations/follow ups from live action to cartoons (Save for exceptions like The Real Ghostbusters and Men In Black) are drastically different from the original.
Back To The Future while fun to a degree feels more like a kid cartoon with the BTTF names and settings. Seeing the cartoon again with exposure to The Original Series, well TAS actually feels like Star Trek.
Actually "More Tribbles, More Trouble" and "Bem" were both intended for the show's 3rd Season. The former was rejected, because they weren't so anxious to do another episode with that particular humor. The latter, because the costume and effects weren't doable at the time.