I really hate to use labels like "favorite". Anyone who knows me can attest to that.
In general, I just like any VS special that has good chemistry between the characters and gives us pretty much everything we wanted to see. Magi VS Deka is my go-to example for this. We got to see the two swordsmen facing off (Deka Master and Wolzard), the main 6 were very similar to each other in so many ways, and the two red screamers of Sentai at the time pretty much had a face off to see who can be more annoying and immature. During the slow moments, where our stars just talk to each other, you could honestly feel some real bonding going on. And even in the fight scenes, there might be moments where they are playing off each other in the best way possible.
When the two teams in question are too dissimilar, the chemistry usually comes off as forced, and then the special becomes all too easy to nitpick and pick apart. The hardest ones to do, especially these days, are the VS Super Sentai specials. GaoRanger and Gokaiger did different things and they all came off great for what they did, but Boukenger's was the worst. To be fair, they tried to do a few things right. They tried to give us a slimmed down version since, let's face it, the older tokusatsu actors aren't getting any younger. They even tried to give us some character interaction and chemistry, but it was all too short lived and very forced (almost to point where characters seemed a little out of character).
It's great when the two teams actually fight each other for a little bit, and we all know they are going to team up against the bigger threat in the end, but I really think the chemistry and those character interactions are the most important thing that needs to be there in ANY regular VS special.