I personally prefer the First Contact divergent point, where in the regular Star Trek universe Cochrane made peace with the Vulcans because the crew of the Enterprise-E had told him they were friendly so he didn't attack. Thus a predestination paradox (those are always fun

) is created with Picard, Riker and co creating their own bright, optimistic future.
Whereas in the Mirror Universe, with no Enterprise-E to meddle in Cochrane's warp flight, an obviously unstable man with a drinking problem, sees an alien ship come out of the sky and assumes the aliens are invading, attacks them, steals their technology and so begins the Terran Empire's bloody crusade across the stars, fueled by regular old human paranoia. Which would kinda make the MU, our universe.
Now alot of people bring up several pieces of contradictory evidence from In A Mirror Darkly to dismiss this theory, but those are actually easily explained away.
Firstly, the opening credits features a Terran Empire symbol over the top of what appears to be Nazi troops marching, and later a person in a space-suit raising the TE flag on what appears to be Earth's Moon. People claim that this is evidence the TE existed as far back as the early 20th Century.
But that is based on the assumption that the credits follow some linear history, which if we compare them to the RU credits they obviously don't, what with overlaying several pieces of historical footage from different time-frames together. Notice how the shuttle Enterprise appears before Chuck Yeager's X-1 flight and a Saturn V rocket is played over black and white footage of Robert Goddard. Clearly these things didn't happen at the same time, just as the TE didn't necessarily happen at the same time as Nazi troops marching through Europe.
Now, the second so-called piece of evidence is Mirror-Archer's little speech to the crew of the Avenger in Pt II of In A Mirror Darkly, when he says the Empire has endured for centuries. I don't know about you, but I'd hardly call M-Archer a credible source of historical data, nor the (possibly) revisionist imperial historians or could just have easily fudged a few numbers for propaganda purposes.
And lastly there's Phlox's little spiel about literature being slightly different (excluding ole' Shakespeare of course) from the RU. Now aside from Shakespeare (who we know was the same in both universes), we don't know which pieces of literature Phlox is comparing, he could be comparing only the last hundred years in which case the universes had already diverged and the differences can be chalked up to that. We just don't know, and in that doubt is the possibility that FC is when it all happened.
Now, before the latest Mirror Universe Novels, specifically Glass Empires: Age of the Empress, the theory of the First Contact divergence was still pretty valid. Obviously, Mike Sussmore disagreed and saw fit to sprinkle some little historical anecdotes about the Empire's history stretching back to the early 21st Century with an Emperor George the Second. While I loved this little piece of satire, it kinda torpedoed my favourite theory on the MU.
But, then again, I've already accepted that some Imperial historians were a little revisionist, why couldn't more of them be, so there may still be a place for the theory. But it's just my opinion.
