Jeff's right. Although no one could ever know for certain who would have prevailed in a straight one-on-one between Nazi Germany and Soviet Russia, the fact remains that throughout the war, Germany fought Russia with only part of its available force, while trying sinultaneously having to bail out its allies in Southern and Eastern Europe (Italy, Hungary, Bulgaria, Romania, etc.) when things went badly for them or they faltered. With the exception of Finland (until late in the war), Germany's allies tended in some ways to be more of a liability than a help.
On the other hand, Russia was fighting principally on one front and able to concentrate practically its entire force on that front. Moreover it received massive material support from the West, not the least of which was 300,000 trucks - trucks that gave the Russians an ability to move and sustain troops and supplies at a rate far greater than that of the Germans, most of whose forces were overwheilmingly dependent upon horse-drawn carts for logistical support.
The Germans may have been much better fighters than the Russians, but the Russians' ability to quickly move vast numbers of troops, equipment and supplies with those 300,000 trucks meant that they could outmanoeuvre at the strategic and operational levels the Germans almost at will, and concentrate forces at a level of 5 or even 10 to 1 against the Germans in critical sectors. With only a fraction of the numbers of trucks that the Russians had, the Germans could not move to counter such Russian concentrations, only to seek to limit and contain the resulting Russian break-throughs and trade more territory for time.
But, had there been little or no Western support for Russia, and had the Germans been freed from diverting troops to garrison the West and the Mediterranean, the Eastern Front would probably have turned out rather differently.