It's hard to make a very good argument that the Finns defeated the Soviets in the Winter War. They put up a good show against long odds, caused the Soviets an embarrassing number of casualties and showed the Germans how hollow the Russian military leadership was after Stalin's purges, but at the end of three and a half months of fighting, the Finns signed a peace treaty that gave 15% of their territory and 30% of their economic resources to the USSR. Pretty feeble kind of victory.
I view it this way;
Stalin tried to make Finland a part of the Soviet Union.
He failed.
This equals Finnish success in a struggle between Mannerheim and Stalin.
The Russians backed off and gave up. I interpret this as a decisive Finnish victory.
What do you wish to call it?