"Mirror, Mirror" (TOS) is my sentimental favorite but it's also very good. But I think the Discovery Mirror Universe episodes do the Mirror Universe the most justice. They're a lot of fun to watch, very visceral, and they don't pull any punches. Though how much you like the DSC Mirror Universe episodes will depend on how much you like Discovery itself. Just putting that caveat out there. The episodes from TOS and Disco are my favorites. Bar None.
Although I do like the DS9 MU Trilogy from Seasons 2-4 (one per season) of: "Crossover", "Through the Looking Glass", and "Shattered Mirror", but that's after the Terran Empire fell.
I like "In the Mirror Darkly" too, and I'm not even a fan of ENT.
The worst Mirror Universe episodes are DS9's "Resurrection" (which doesn't even take place in the Mirror Universe but has an MU character) and "The Emperor's New Cloak", which I consider to be the bottom of the barrel. For lack of more colorful language.