It is well known whom Iran declared the Little Satan and the Great Satan. They are Israel and US respectively. But it is little known whom it considers the greatest Satan. Meanwhile in politics what is not said is much more important than what is said. So, who is the undeclared greatest Satan for Iran ?
For the last 200 years it was Russia, which captured from Iran Dagestan, part of Azerbaijan, Eastern Georgia, part of Armenia, Central Asia, etc. and imposed Turkmenchay peace treaty, which also prohibited Iranian ships from sailing the Caspian. The Soviet Russia went even further. On the one hand it canceled the Turkmenchay peace treaty in 1921 but on the other hand it replaced it with the Treaty of Friendship. The "cancelation" did not give back to Iran any territory captured from it by the Imperial Russia. Only sailing the Caspian was canceled on the paper. But in the reality the Soviet Navy guarded its marine frontiers and did not allow Iranian ships in. In exchange for "cancelation" of Turkmenchay treaty the Soviet Russia got the right of entering Iran with force at will if Russia or its allies (i.e. other Soviet states) feel to be threatened from the Iranian territory. That was this article of the Treaty of Friendship that Stalin invoked when sent divisions to Iran during WWII.
There is no Soviet Russia anymore but the new Russia is its heir and, thus, the Treaty of Friendship is still in effect.
According to some sources Iran considers the Turkmenchay treaty, treaty of Akhal, and the Treaty of Friendship as the greatest humiliation in its history. So what will it do when gets strong enough and acquire nukes ? Will it attack Israel or US ? It is more likely that it will first denounce the humiliating Treaty of Friendship with Russia and then annex Azerbaijan...