I think I'd go with DS9 "Let He Who Is Without Sin." You can't really top that for unexecuted potential, in that the entire episode is supposed to be about sex, yet it's so chaste you can't even have a man shirtless at the beach. There's no point in doing a completely neutered sex episode.
IMO that episode was less an episode about sex, than an episode about our main characters indulging in
sex tourism.
Hard to say there was any unexecuted potential. Probably only if they had decided to go
really ammoral anti-hero with our main characters. As it was, it was just embarrasing. Both in execution
and original potential.
"Birthright Part II" over on TNG would actually be my runner-up -- another shitty installment that totally trashes Worf! He is so awful and racist here, and it's not worth it because they fail to do anything dramatically interesting with his racism. And I loved where Data's story had left off and was DYING to see what happened next with his dream exploration, and nothing. Honestly I think I was 35 minutes into the hour before I finally admitted we weren't doing more with Data. AND they couldn't do another scene with James Cromwell's awesome Yridiian character because he injured himself! So many failures.
Worf was ALWAYS a self-absorbed asshole! That's part of his appeal. That really
everyone can get along in the future. Even the big dumb self-absorbed klingon can be a precious member of the society, and work together with the "nerds" and do good. (Being the only single dad on the show fits were well with his douche-bro appereance.

) Him facing his predjudices against Romulans wasn't the first, nor the most egregious time.
Totally agree about Data, though! I really thought this was the beginning of an
awesome character arc! Instead, we just
never, ever got a follow-up on. THAT was truly disappointing, and so so much unexecuted potential wasted.
"Fury" on VOY is another good one. This is dead on:
I was no longer regularly watching Voyager in season 6, but was super excited when I heard Kes was doing another ep and made a point to watch. It really would have been better to not bring her back at all, "The Gift" was a perfect character exit, which "Fury" retroactively destroyed while adding nothing.
Kes was Luke Skywalker before Luke Skywalker was the Last Jedi!
Seriously though, I loved that episode.
I loved seeing Kes again. And I think her journey was a believable one. Her leaving on "The gift" was always an ambigious one - both bursting with potential, but also with danger. It was very fitting with the tone of later Voyager that not everything worked out perfectly fine. And Kes didn't turn out flatly
evil - just misguided. Which I believe can be disappointing for a fan of the character. But it fits very well with her journey, how naive but still strong-willed she was in the beginning and still is, just without the youthful optimism. It's just a more tragic story. But ultimately, she still was able to find back to her true self. She just wasn't capable of handling to be completely alone with the unknown and her expansive powers for most of her life. She
needed the compassion and friendship of others. Which she didn't get that in her new plane of existence. I love that story. It's a sad one, yes. But also a much more complex and intimate look at her character than we ever got before.
Overall though, I think "Fury" was more of a story for the fans still watching Voyager, in the style of the later series, with more complex and darker themes, and a nostalgic look back at the early seasons. And
not an episode to be singularly watched by the people who left early on and were expecting a small, friendly greeting of a familiar character to show by years later.
The only part I didn't like about that episode was that the time-travel/predestination paradox, where Janeway was retconned to
know something will happen years later in the timeline (like in 'Relativity', where she was retconned to know about Seven of Nine years earlier), because that takes a bit of the punch out of her original decisions in those cases.