7. Through The Looking Glass (Star Trek: Deep Space Nine)
The Mirror Universe episodes of Deep Space Nine are brilliant. Because of the nature of the Mirror Universe, the potential for including characters from other Star Trek series were always there but this episode was the only one that took up that challenge. If you are going to watch this episode then don't blink during the course of it because you'll miss Tuvok from Star Trek: Voyager. It was the second episode in the Mirror Universe arc within the series, ironically the first was called Crossover but didn't crossover with any other Star Trek series. But he isn't the fun part of the episode, because as with any Mirror Universe episode it is to see which evil versions of the characters get introduced and which ones get killed off. In fact, it almost became a tradition to have a Ferengi die in each Mirror Universe episode. Some of the Mirror Universe characters are a delight, like the Intendant and this particular episode is where Miles "Smiley" O'Brien comes into his own. Through The Looking Glass which is an excellent episode although you do rather need to watch the other episodes of the show first in order to get the full benefit.
