“Who Watches the Watchers” is self-contained but you can also show someone “The Drumhead” after they’ve seen it. Beyond that also being a good episode for newbies as suggested elsewhere in these threads, the callbacks to “Who Watches the Watchers” nicely illustrate how threads from one story can keep affecting and enriching the show’s universe.
On the big wall of TVs at Cyberdelia, the nightclub from Hackers.