2x03 The Waldo Moment [season finale]
This is by far the lowest-rated episode of Black Mirror according to IMDb users -- whereas all the others range from 8.1 to 9.0, this has 7.1. And they're not wrong. There are some interesting and sometimes amusing ideas, and like all the best Black Mirrors it's sickeningly plausible, but it ends rather abruptly without seeming to have reached an ultimate point, and a future-set coda gets kind of silly. Ah well.
Duck Quacks Don't Echo
5x07 Episode 7
5x04 The Dreaming Dead
[Watch it exclusively on Amazon Prime Video.]