I didn't expect this, this feedback is BAFFLING. Plato's Stepchildren is a great episode. Plenty of tension and an extremely hateable antagonist, possibly the most opposed I've ever been to a ST character. You just want to see him get crushed so badly but then Kirk lets him go after getting the upper hand, which is really admirable and noble. Kirk rarely makes me like him quite this much.
The acting is some of Kirk's very best, Spock's is excellent, and Uhura and Chapel are also great. The dialogue couldn't have been more appropriate.
There are a couple exceptions. THE ONLY bad things about this episode: the fact that the food gives them the powers and Kirk is just fed the stuff is questionable, and the way Kirk treats the dwarf like he's a child near the end (and the way he uses the dwarf against the antagonist). Incredibly disappointing stuff in an otherwise perfect episode. It's bizarre that you guys all failed to mention the worst parts about this episode and then go on complaining about great scenes.
Alexander riding Kirk like a horse was at once hilarious (the image, neighing), chilling (just the thought, reminds me somewhat of Caligula and makes me wonder how far this is going to be taken), and even awe-inspiring (I just can't believe how well it's acted). Watching them doing all of these things against their will shouldn't look so good but it really does seem as though they're being forced against their will to do everything they're doing while trying to fight it. Think about how corny and stupid that should look. Imagine trying to do that yourself, that had to be so difficult. It's also great to see Spock getting emotional for no reason, what an actor! It's also just very TOS to find an excuse to get Spock's human side out. The music is also suspenseful and perfect. You're all out of your minds. The only season 3 episodes surely better than this: The Enterprise Incident, The Tholian Web, and Day Of The Dove.
Is There In Truth No Beauty?, Wink Of An Eye, Requiem For Methuselah, and All Our Yesterdays are arguable as well, but that's about it.
You put too much stock in silly mental currencies like pride, honor, respect, etc. There is no "shame," grow a spine and get real before rewatching Plato's Stepchildren.
The other "bad" eps you guys are talking about too, just "lol" is all I can say. A Piece Of The Action, I Mudd, Patterns Of Force, Spock's Brain...? Get a sense of humor, what's next, Trouble With Tribbles sucks? Turnabout Intruder is also awesome and well acted, it really shouldn't seem like Kirk and Janice are in each other's bodies but it does and the criticism that it's "sexist" is nonsense. That's just the way Janice Lester is. One person can have any non-contradicting combination of traits and the fact that you think her combo is "sexist" just proves that you believe her traits are stereotypical of women. That's on you, not TOS. If 1 male character on the show had those traits, the word "sexist" wouldn't have passed through your mind and you know it. She's a formidable antagonist and giving the crew reason to mutiny against Kirk is one of the more interesting plot points of the series. To say it's one of the worst episodes is a joke, it's far from boring and there's a boring episode or two.
An actual "bad" episode (for TOS) is That Which Survives. Relatively predictable, little characterization for anyone, repetitive, few memorable lines, unoriginal plot, played out. At least to the best of my memory. I just tried to find one I didn't like so that someone can get me back for what I said and hopefully give some reasons to like That Which Survives. I'm pretty sure it was boring though.
EDIT: I forgot to comment on the opposition towards The Savage Curtain. Lincoln was the best choice, the others you mention are slave holders or imperialists, the great emancipator just makes the most sense as Kirk's hero. Being pulled from Kirk's mind, it's completely justified too, the episode isn't ridiculous at all.