(Beyond the spoilers is a small summary of these episodes which doesn't give away too much but I tend to think that going in completely blind is the better experience. So in case you want to know a little bit what the episode is generally about you can safely read these.)
Chiming in on this, just adding to the previous very good answer, I really liked "Beyond the Sea" S6 E3, the one with Aaron Paul.
It's about astronauts who can connect with their family on earth by some kind of android which they can control. However, this goes horribly wrong at some point, as all Black Mirror episodes tend do to.
Also one of my favourites is "Black Museum" S4 E6.
There are 3 little stories woven together in one episode which on their own are already great. Memorabilia of these objects are being displayed in an eery museum which serves as the connection between these stories.
Lastly, I somehow enjoyed "Crocodile" from S4 E3.
It starts with an accident which becomes a secret for the two people involved. Years later, an insurance investigator investigates another accident by tapping directly into witnesses memories.