Honestly this is half for that first part alone, that's genuinely haunting, it's like The Happiness Patrol dialed up to eleven.

The worldbuilding and haunting imagery that comes with it would already be enough for this story to be great, The Doctor bringing emotion back to an emotionless world is a great premise, but then it goes and does even more with it than I'd expected.

The ZEPO are admittedly a little silly, but Doctor Who is at it's best when it's silly, and I think they're a really interesting part of the story. A people who've been forced to live without emotions, knowing that they want to feel, but not understanding how to, so each of them choosing just a single emotion to take on and keep alive. It's a great bit of worldbuilding that paradoxically makes somehow makes the resistance feel even more real through how cartoonish they are.

And then they use this as a commentary not only on the need for emotions, but the pain they can cause to others as well with Big Hate releasing essentially a biblical plague on the city out of frustration towards the moderators. The Doctor gives people back their emotions which conveniently stops it, but in that attack so many people are killed, there's a haunting panel showing how for some it came just too late, a man violently squishing the bugs as his wife's skeleton is covered in them in the background. Imagine your first emotion being that mix of grief and rage, it's just brilliantly written.

The black and white art style as well works perfectly, I've had a quick look at some of the colourisations, and while I think it adds a little to The Iron Legion, it just takes away from this story, the almost clinical nature of the monochrome art adds so much to the world of the story.

Also couldn't find anywhere else to mention it, but the designs of the Brains Trust are just so good, I love it so so much.