Russia. My strategy is to pretty much buy as many tanks as I can in Germany. Load some infantry into Eastern Europe, and take Karelia on my first turn. If they take it back I boot them out with the 8 tanks from Germany and then I can smash into West Russia, forcing them to either lose Russia or Caucasus. Either one loses, because after I take one I can always continue with my tank chain from Germany through Eastern Europe to Karelia to West Russia, and then I can hit Russia with 15-22 tanks. I seriously doubt Russia can build up enough defenses to stand up to 15-22 tanks. Just in case I have really bad luck with the attack, I pretty much go undefended transport hunting with my subs, and with Japan, you can usually end up blitzing a tank through the entire eastern Russia without much defense.