The most important territory?

[88 Millimeter]

I'd submit the deadly combo of West Russia and/ or Kazahk, the Russian backside equivalent. Both of these territories command both Russia and the Caucasus. Typically both can't be defended. If Germany gets embedded in W Russia, or Japan gets its meathooks in Kazahk, you're looking at a puddle of pee under the Allied player's seat.

[DarthMaximus]

I'd probably vote for WRus.

As 88 points out the Wrus/Kaz occupation is a game ender.

Wrus commands all of Europe.
While Ukr is nice, it does not necessarily force a retreat for your opponent.
The Allies still could face a giant stack in EE and the Axis have to deal with Allied stack in Wrus and Cauc or possbily Kar.

If the Allies hold WRus they can go from Kar to Cauc with ease, can control Arch as well and still put immense pressure on Ukr, shuffle troops to Mos.

But if the Axis hold Wrus (assuming Japan is playing up to par), that should force the Allies out of Cauc and deadzone Kar and Arch as well, cutting off Moscow.