2 inf 2 art 2 tank is the classic buy for the Ukraine/West Russia attack.
You hit West Russia and the Ukraine. You will lose the Ukraine to the German counterattack for sure, but if you put 2 inf 2 art in Caucasus and 2 tanks in Moscow, you can counterattack Ukraine with the West Russia stack, the Caucasus units, and the Moscow tanks. Now - if Germany attacks Ukraine lightly, you can retake the Ukraine with a few infantry and a fighter. if Germany attacks Ukraine heavy, you can smash the Ukraine.
Germany can re-counter with a pure tank build, but that is not sound long-term strategy.
Germany can also counter by attacking heavily into Karelia - at which point the Ukraine stack hits and runs Eastern Europe and Balkans with infantry and fighters, and the Ukraine stack moves to West Russia (reinforced by units from Moscow), at which point the game transposes to the usual West Russia / Eastern Europe game.