I've said before, that for me, the three things that make the cyberman (in theory) my favourite doctor who villains are: Inevitability, Conversion, and Unstoppability.
This story makes use of all three.
The story here, it's revealed eventually, is the result of a single, lone cyberman crashing on a planet, and plotting to take it over through guile rather than force. In Cyberwoman Jack implies that even a single cyberman could be disasterous for humanity, and here we really see that. Intelligence and manipulations aren't often something that's focused on in Cyberman stories, but the way we're shown here how a single one can use a culture that's already forcing repression of some emotions... It's just works. We're also shown that culture, the comparisons between something that's happening here and now to cyber conversion is terrifying, and I think satisfies my inevitability criteria.
As for unstoppability, again, this cyberman crashed, but built itself back up to become a major power in this world. Even a cyberman that's been seemingly defeated will get back up and try again.
But all of this is missing the forest for the trees. What makes this story isn't the cybermen, it's the people and the setting. A conversion camp being a hiding place for the cybermen, cybermen using faith and belief to infiltrate. A lot of the horror, for me at least, of this episode isn't even the cybermen themselves, it's the existential conflict that the people in this camp are going through, the shame in peoples voices as they talk about understanding their sexuality, all watched over by a cyberman telling them that it's okay, they can be freed from those feelings. The fact that even after Valarie shows them what the cybermen are, they all still want to go and be converted because they can't face themselves. Carmen when her emotional inhibitor is turned off, fighting against her dad for what he's turned her into. Conversion.
I also love what this story does for Valarie, after the events of the previous story, a story putting sexuality and beliefs at the forefront just makes sense. And I love how even with the cybermen all focused on what The Doctor's plan is at the end, it's really Valarie that saves the day with her mind and body (literally).
Plus again, I think my favourite part of these stories is that even after the 'story' is over, the conflict is solved, we don't just whizz off in the TARDIS until 'next week'. We're allowed to sit. We see Lilly confronting her family in a scene that reminded me of The Idiot's Lantern, an extremely favourable comparison. We see Valarie talk to The Doctor about how he's been looking at her, and him opening up about Clara and the Ponds. Those character moments where we can just feel the repercussions of what's happened are what makes this series my favourite Eleventh Doctor season.
