Emperor's New Cloak, no contest. It's the only one that I enjoy even a little. I thought it was a fun trip, and enjoyed bizarro Ezri. It's the only MU ep that...really, is in any way entertaining, which is due in part to having a lot of comedy in it. I also thought there was a great Quark moment in this one, showing that he does have some loyalties where he tells MU Ezri, "There's nothing to understand. He's my Nagus."
I think the MU is a stupid idea to begin with, and it's hysterical how TOS, ENT, and most of the DS9 ones play it seriously.
I think that this is exactly the mistake that DS9 writers did - believing that the Mirror Universe is a silly idea that should only be used for comedy, overacting, dressing up, girl-on-girl action and silliness. This is what lead them to turn the MU into crap, as seen most obviously in the horrible episode "Emperor's New Cloak". And don't get me started on the whole "evil bisexuals/lesbians" thing...
Mirror Universe, as seen in TOS "Mirror, Mirror", is actually a brilliant and very interesting idea, and can lead to wonderful stories if taken seriously - that in a parallel universe, under different circumstances, our heroes could have developed completely differently, that instead of peaceful explorers, the human race and its allies could have become ruthless conquerors, and that the people we know could not be the same if they grew up and lived in a barbarian dog-eat-dog world. Terran Empire is basically the dark side of the Federation, of humanity and of the heroes of the show. ENT "In A Mirror, Darkly" elaborates on that. DS9 "Crossover" threw an interesting twist - the idea that Spock's reforms of the Terran Empire lead to Humans becoming slaves, and it was suitably dark and creepy - though I always had the problem with the Intendant, since I could never buy her as an alternate, dark version of Kira, only as another character played by the same actress. If only DS9 had stopped after that episode, or after the first 2 episodes... the MU on DS9 later turned into a joke. And, really, since the Human rebellion started, the MU lost its point. It all became a 'rebels vs evil empire' story with Humans as good guys fighting against evil Klingons and Cardassians. Well, how original, we really needed a parallel universe for that.
ENT's have grown on me, I do like seeing Archer in the green shirt and the set of a Constitution class starship. The tie in to TOS with the Defiant was excellent. But all machinations and evil hissing of the MU crew was tedious. Yeah we get, you are eeeevil..
And that is different from all the other MU episodes, how?
It's between "Mirror, Mirror" and "In A Mirror, Darkly" for me, but I voted the latter. I'll quote myself from another forum: I like that it's 100% a Mirror Universe episode, even down to the new opening credits and the
awesome theme, with no characters from the prime universe and fully from the POV of the MU people, and that it's so bleak (which, of course, it had to be, due to the time period, since we knew that the Terran Empire would only get stronger from that point on).
Mirror, Mirror and
Crossover, when you think about it, fit the same format: our heroes cross over to the parallel universe, are shocked at finding out about their 'evil' counterparts, and before they leave they affect the people in the MU universe and inspire them to rebel, and it ends with a message of hope that things are about to get better and the oppressors will be overturned or at least challenged.
In A Mirror, Darkly features people from the MU finding out about the Prime Universe, but what they find (The Defiant) only helps the Terran Empire become stronger, and it serves the most ruthless people go their way to the top. It does also inspire some people (T'Pol) to rebellion against the Empire, but the rebels (the only faction that we could feel some sympathy for) meet a sorry end - with just a prophetic line by T'Pol to suggest that the Terran Empire would fall one day - and the battle for power between Humans is all about who can be the most ruthless and scheming. Which all makes it the darkest of the MU episodes.