The episode that started it all is still the best mirror universe episode of Star Trek.
Kirk, Scotty, McCoy and Uhura find themselves in the parallel universe thanks to a transporter malfunction when trying to return to the Enterprise after speaking with the Halkans. They notice things are different pretty quickly since their clothes have changed, everyone starts saluting and Spock appears to have grown a very sinister-looking beard! They head to sick bay, where they figure out they must be in another universe and have to find a way to get back.
These strangers in a strange land try to blend in as they come up with a plan, but Kirk's delaying the destruction of the Halkans in this universe starts to make Spock suspicious and soon it's a race against time to get back before Spock is supposed to kill Kirk and deal with the Halkans himself.
Along the way Chekov tries to assassinate Kirk and ends up in the Agony Booth, Uhura has to distract a creepy Sulu and Kirk discovers he's been considerably successful in this universe because of the Tantalus field, a piece of technology that vaporizes people.
After the four fight Sulu and Spock, they're almost able to escape with the help of Marlena before being delayed by both her and a recovered Spock. However, it turns out this Spock wants his captain back, so he's willing to let Kirk and the others go. They return to their universe ... where a beardless Spock waits for them.
While little things like the characters changing clothes with their counterparts don't make a lot of sense, this episode is otherwise amazing. While some of the 'evil' counterparts are quite stereotypical, they're also a lot of fun and let the actors play around with the characters in new ways.
Ultimately, the mirror universe with its alternate history was extremely creative ... and not a place easily forgotten.
Best scene: Kirk's speech to Spock
We know thanks to Deep Space Nine that this speech had quite an impact on Spock and did end up changing the fate of the Empire. Before leaving the mirror universe, Kirk speaks to Spock about how illogical and wasteful the Empire is, and since the Empire will fall eventually anyway Spock should change the present. He gives Spock the power to do so by leaving him the Tantalus field and leaves the Vulcan with the idea of freedom over tyranny. Spock says he will consider it ... and, as we know, he does and acts upon it, leading to many changes in the Empire. It's a great speech delivered by Kirk and one of his most memorable.
Share in the comments which mirror universe episode or moment is your favorite!