For a variety of reasons (equipment, philosophy of warmaking, fear of Stalin) the Soviet forces remained vulnerable to fluid, mobile warfare, even after they perfected the set-piece offensive (at which they became very good indeed).
Perceiving this, Manstein and other German commanders believed the way to fight the Soviet army was to never remain in place and bear the brunt of a Soviet attack, but to always disengage and maneuver. Sooner or later, during maneuver warfare, Soviet logistics might fail, the forward elements might lose contact with the higher authorities who made the real decisions (and thus become paralyzed), the battleground would move away from pre-planned artillery fire zones, and so on. Then German forces could turn on the strung-out Sov columns and hit weak spots.
This method worked pretty well the few times they were able to get Hitler to approve disengagement (which he thought of as retreat), or employ it without Hitler realizing. One of the key reasons the Germans collapsed when they did is that Hitler usually forced them to fight in ways that played to Soviet strengths, not German strengths.
As it was, the Soviets relied heavily on American aid for transportation, fuel, clothing (5 million pairs of boots alone), and food (the US sent enough food to feed everyone in the Soviet military for every day of the war) just to conduct their set-piece offensives. Nikita Kruschev famously said something like “Imagine the advance to Berlin without American trucks!”
IF we postulate that the German generals could have somehow been freed to conduct mobile warfare, it would have been MUCH more effective against a Soviet army without American logistical support and transportation – a substantially less mobile army.
So I guess my take is, without the Western Allies, the Soviets would have been surprisingly vulnerable (despite their massive military) to a technique the German generals wanted to use, but did not get much chance to use. If we assume Hitler let them use that technique, or choked on a potato in 1942, it becomes possible to imagine a German victory, or a partial victory.
We know Stalin was fearful of defeat early in the war, and it must have been a psychological comfort to have received promises of support from the West. Without those promises, without those supplies, with his immobile army slashed up by fast-moving, elusive German forces…might a hypothetically-cowed Stalin have agreed to quit while he still held the eastern half of the country, leaving Germany holding the western half?