I would say its a tough choice.. being that the revolution was a critical turning point. But it alone did not define russia or USSR...
The defining moment came when the Germans invaded Mother Russia.
Russians/Slavs/kosaks/Tatars/Ukrainians all came together punting aside there differences, welding the national identity and making Russia (USSR) a great power again.
Now of course this fell apart later on due to mismanagement from the top, but no event shaped and brought the eastern Slavic people together like world war II.. it was fight or die...
Revolution only deposed of the old guard and brought Kerensky at first.. then Lenin, then civil war, so obviously many amongst the populace were not thrilled with the ideas of Lenin LTD.
( Tis funny how almost 100 years later, the Romanov's are looked at in a nostalgic light, but from '05 - '17 onward the masses were not pleased with his autocratic highness Czar Nicholas II. they even canonized him a short time ago.. go figure.. he screw it up so bad for the family that Mikal, was like.. uh.. no thanks.. i prefer to not be dead.. )
People did not fight for comrade Stalin.. they fought for MOTHER RUSSIA and their lives.