Most important territory?
I’d say either Persia or Novosibirsk. Reason is, if the Allies can have a strong stack in Persia it really distracts Japan and secures Africa. If the Allies have a really strong stack in Novosibirsk it pretty much secures, forever, Russia.
Just my impression. Could be wrong.
As far as Sea Zones go, I’d say SZ 6 is the most important. If you’re strong enough to sit your fleet there, you’re already technically in possession of SZ’s 5, 3, 4, 2, 8, 7 and 12 and by extension the entire Atlantic Ocean. If you are not strong enough to sit your fleets there (as the allies) then you are losing to Germany, even if Germany does not sink your fleet.