I would pick Russia.
Yeah, I would too, but it would be a lot tougher than some seem to imagine. On the plus side: Russia will have fewer resources to invade Estonia if they’re having to tie up tens of thousands of troops in Turkey.
I don’t see Russia attacking Turkey right now. They don’t have the fig leaf that they have for Crimea and Estonia (ethnic “Russians” already living there because of the Russification during the Soviet days). Going after Istanbul and the straits would strip the mask, and I don’t think Putin is ready to do that just yet.
Here’s the “tell”: If Istanbul denies the Russian navy access to the Med, and Putin calls that an “act of war,” then he’s ready to invade Turkey. All Hell will then break loose, and NATO will be forced to take a stand.