re: Best ever series episode?

A lot of options to pick from;

GoT- Hardhome was great. Tyrion verbally jousting with Dany, and the Night King wiping out the Wildlings. My brother hasn't watched this series, and he sat and watched this episode with me. He commented that singular episode was better than a lot of movies he'd seen, in terms of epic battles and intelligent dialogue.

Lots of older series have epic moments-
WKRP in Cincinnati had some brilliant episodes. Thanksgiving (Les Nessman's on-the-scene reporting of the turkey drop was SO awesome )
The phone gestapo episode was gold too!

If you count Band of Brothers, then quite a few individual episodes match any war movie.