The defeat of Hitler and the Nazis was due to several simultaneous factors, including the Allied powers, various resistance movements, and Hitler's own strategic mistakes. Thus to ask for an either/or answer is to assume something simpler than was actually the case.

The Allied powers fighting against Hitler included the United States, Britain, Canada, China, and the Soviet Union (of which Russia was part). The US and Britain operated on the "western front", conducting an air war against Germany and invading and liberating much of Europe. Heroic resistance fighters in several European countries aided the Allies.

The Soviet Union was the largest country on the Eastern Front and was also instrumental in Hitler's defeat. Sometimes it is argued that the Russian winter itself was a major factor in Hilter's defeat, as the German soldiers were inadequately equipped for the harsh conditions, and many succumbed to frostbite and hypothermia and even those who did not die directly from the cold were not effective in fighting in extreme winter conditions.

Many military historians argue that Hitler's own bad judgment towards the end of the war contributed to his defeat, especially attacking Russia while Britain remained undefeated, leading him to fighting on two fronts, a position that was unsustainable.

Yes, the Soviet Union did much if not most to bleed Hitler's Germany and finally overwhelm them, but in the end, I would have to give the most credit to Hitler himself.  He made extremely poor decisions (such as invading the Soviet Union, for one) and even though his military experience was very limited, acted as though he was a brilliant warlord.  In so doing, he led his country and his people to ruin.

I would give the most credit to the Soviet Union.  By far the majority of German military men who died in the war were killed by the Russians.  Only the Russians were seriously fighting Germany on land from early 1940 until 1943.

Of course, the US helped by giving weapons to the USSR and by the invasion of France and such, but the USSR really did the majority of the fighting.