I'm not so sure I'm as in love with "Yesteryear" as others here. While it would be nice to see Spock's childhood on Vulcan in a live action show, the setup makes no sense: Spock was busy in Orion's past, so he wasn't available to go back into his own past and become his "cousin" as he remembers it, so the timeline was changed. But there was no motivation for Spock to visit his younger self in the first place; he only does so in order to set things back to normal. Or to put it another way, there was no "first time" that could have been screwed up "this time". Even the sehlat's death puts another crimp in the logic of the story, since there was no "first time" where he lived.
My first vote goes to "The Slaver Weapon". While it would be a crossover of sorts, there is no reason the two canons need to be merged, just the aspects of this episode. We'd have to find a way to include Kirk, and it would be nice to also include a puppeteer, even if Spock takes the place of Nessus in the story.
Others I'd like (although there are less problematic story issues here, too):
The Jihad
The Magicks of Megas-Tu
The Pirates of Orion
The Survivor, if it meant getting Ted Knight on Star Trek