I always thought that penultimate slot had an unusually high concentration of peaks and valleys, and this list seems to prove that. A surprising number of all-time favorites on here.
I'm not often moved to save TOS, but I'll start with "All Our Yesterdays." I just love this one, it's such an odd but well-done idea. And it's dying planet setting is the perfect match for that TOS energy and vibe. I also think it's a great Spock story.
About my only complaint is that for the 2nd to last ep of the series, Uhura, Chekhov, and Sulu are completely benched, and Scotty just has a bit of voiceover. But, that was always my overall complaint about TOS -- it would have been greatly strengthened by being more of an ensemble.
It's a gem. Jean Lisette Aroeste puts in a cracking, refreshing script that doesn't feel like "last drawn out entry of a last season of a show everyone stopped caring about." It has a couple quibbles, but even the worst can arguably be explained*. How Spock "devolves" is subtly and suitably done. Jean definitely knew how to handle Spock and Bones. kirk too. The ending we sorta knew from the start would go "boom", but that wasn't the point. The structure of the story readily sells the drama and sense of threat and is well composed. Definitely a must-see episode.
* only Spock devolves due to the de-evolution, presumably brought about by not being processed by the Atavachron. Nobody else devolved, maybe it's a Vulcan-centric issue and I'll buy into that, though what might it have been if McCoy and Kirk started devolving too? A missed opportunity, but that would impede the pacing - which this episode handles beautifully.
Arrived late, best ones gone, but I will save "Worst Case Scenario", a fave of mine. The first half anyway... the second deteriorated into another "holodeck malfunctions and people are in real danger" plot. Still, the first half alone makes it worth a pass.
WCS does drop the ball, perhaps too quickly, and the more the episode goes on the more the contrivances beg the viewer to get annoyed. But it doesn't quite fall apart despite it all.
I'm going to save DS9's The Sound Of Her Voice.
This episode always struck a cord with me. The way that during a time of high stress, the crew is daring to show how vunerable and hurt they are. How they feel they are cracking. And how a connection with a person they never really knew, connects them again. I loved that.
I also really loved that short little shuttle bay CGI.
A great setup and execution, it's topped with an ending that's a real belter. On an initial viewing (spoiler alert for a 30-something year-old show

), the twist that there's a Temporal Treknobabble that has the sole survivor in a one-way different time frame whatever and, in Sisko/O'brien's timeline she's been long since dead**... Debra Wilson puts in a performance earning much respect. She instills humor into her character, but without doing what many comedic actors might do and go over the top. I wanted to see her character at the end... but compared to other shows where hack writers set up a character we want to love and then kill 'em off for a cheap shot (often surrounded with a big f/x piece with loud muzak), Lisa Cusak's backstory and fate definitely feel earned and definitely do not feel cheap. This episode is proper robust writing and casting. The fact they held off the treknobabble until the end must have been a conscious decision. It is well-timed and presented, making it more than the sum of its parts.
Lastly, the story's title always reminds me of a song with a not-too-dissimilar title:
Yes, I'm weird.
** dayum, that was a brutal twist, to cut a story short...
(oops, I did it again...)