19. Men Against Fire
Despite being down at the bottom here, there's nothing really "wrong" with Men Against Fire. It's a competently put together episode with decent performances and a couple of exciting sequences. The problem is that, for a Black Mirror episode anyway, the central hook doesn't have all that much to it.
It's easy to guess that the supposed alien "Roaches" the military main characters are being sent out to kill aren't going to really be alien at all, but rather human "undesirables". This theme itself is certainly relevant and worth diving into - and the idea of regular soldiers being manipulated to make genocide seem like a game has formed the foundation of so many other great stories - it's just the episode is so heavy handed with its execution that the overall message comes at the expense of compelling storytelling.
In a way, you kind of wish Men Against Fire was a little worse, then at least it would elicit feelings stronger than a shrug of the shoulders.