I think "Mirror, Mirror" would be a good candidate. Imagine if they'd beat ENT to the punch and done an entire episode or two featuring just the mirror crew. We were robbed of seeing Mirror Kirk in his element.
"Mirror, Mirror" is the one that intrigues me the most. One of my few disappointments with the original episode is that we never saw exactly
how Spock realized he was dealing with the Kirk, McCoy, Scotty, and Uhura of a parallel universe. We know that they were discovered relatively quickly, and we have Spock's explanation that "they, as barbarians, were unable to act like civilized men." which doesn't tell us anything in the way of specifics. Even as a kid, I thought that that was a bit of a cop out.
I have some ideas of what happened next, but I'll keep those to myself. They'd start getting too close to the "don't post story ideas" rule.
But I've enjoyed pretty much all of the Mirror Universe sequels to some degree (Mike W. Barr's "Mirror Universe Saga,"
Dark Mirror, the DS9 episodes, ENT's "In A Mirror, Darkly," David Mack's novel,
Star Trek Continues, and John Byrne's recent
New Visions story), even when I might disagree with them.