I’ve been watching it since it hit Ch4 in the UK and my favourites of each series are probably -

S1 - Fifteen Million Merits (honourable mention to The National Anthem for going so hard with it’s first episode content and theme wise)

S2 - Probably White Bear, but White Christmas is also good but was shown on its own over here rather than part of S2 as it is on Netflix. This series is hard to pick from too be fair as all three EPS are strong in different ways.

S3 - Shut up and Dance is probably best for your chreepy, bleak fix. But honourable mention to San Junipero for being the first feel good episode they did

S4 - You’ve already watched USS Calister, I’d say Metalhead is your next best. Points also to Hang the DJ for an interesting look at common tech in use today.

Bandersnatch - Play this, without a doubt. Charlie Brooker knows his games and it shows in his own choose your own adventure type experience. Some of the hidden endings are amazing and hilarious.

S5 - Striking Vipers, an amazing exploration of sexuality and identity and what happens when the consciousness is decoupled from the physical.

S6 - Haven’t watched it yet.