Who finally defeated the Nazis?

Allied bombing of Germany day and night but primarily the Red Army on the eastern front driving German army out of Russia back to Berlin.

The Soviets mainly

nazis defeated themselves with extremely poor planning.

Russians

Adolf Hitler. He killed their leader with a one tap.

Since the USA never had a peace treaty with NAZI Germany the war is still on. And the NAZIs have never been defeated.

It's said some 70% (or there abouts) of all German causalities were on Eastern front with Russia. As mentioned in other answers The Allied Powers defeated Germany ... Nazism is an ideology (even though it became a dominating one in WW2 German) so I'm not sure if an ideology can actually be defeated, just marginalized or co-opted.

Principally the Russians

The world.

Peace.

Russia ,USA and the Allies