Remember, kids: two 24th century foes at the height of their empires. So for the Cardassians, that probably means pre-Dominion Occupation, and pre-destruction of the Obsidian Order.
I'm torn (though I kinda wanna say Romulans; I mean, we didn't see the Tal Shiar get completely destroyed). I mean, the backstabbers versus the other backstabbers...who'd win?