I know counterfactuals are essentially circlejerking, but I'm jerking away anyways.
Early in 1943, furious at the western nations that he feels brought this war upon him, Stalin orders preliminary preparations for a surprise attack to follow as closely as possible behind the German surrender as possible- waiting only if there's a clear advantage to be gained, such as veteran combat units being swapped out. A special STAVKA task force is created to monitor the situation and prepare.
Stalin orders loss figures and lend lease requests to be wildly inflated, portraying the soviet union as an inept, spent force, advancing only by sheer weight of numbers, running on fumes metaphorically and literally. Any extra lend-lease they get from the frantic pleading is a bonus.
Stalin himself puts on a masterful performance of putting the allies at ease, making concessions at every negotiation, complying with every request, even throwing his full support behind the Warsaw uprising and installing the Polish exiles in control with the understanding that they are expected to contribute to the war effort. His paranoia is put under control by the knowledge that success requires total deception.
When the attack comes, it's completely unanticipated. Rear area troops have been under intensive training that only now makes sense to them, and they no longer wonder why they're so well-supplied for rear-echelon units.
Having learned the lessons of earlier campaigns, his armored divisions leapfrog each other continuously, stopping to refit at designated points based on failure rates, mean component lifetimes, their objectives, etc.
Key points like railway junctions are already seized or by elite troops and infiltrators with the assistance of large bodies of communist sympathizers. Some of the railway network is damaged, but not enough.
Atomic weapon use is eventually authorized after great debate, but the soviets make it known that they will retaliate for any nuclear attack by "executing captured war criminals en masse" (allied pows) or "attacking legitimate military targets in England" (terror attacks, including dirty bombs after some inquiries in Japan).
Besides, by now the nukes would have to be dropped in France, Spain and Germany to have any immediate effect.
The offensive stops after a defensible perimeter is established. They don't need to hold every inch of ground in Europe, just enough to show clearly that invasion would require a massive effort. They even release a full analysis of the military situation, showing the estimated cost and loss rates of a seaborne invasion, of strategic bombing, the casualty rates and fallout patterns of a nuclear strike, etc, and invite the world community to dispute any of it.
The pows are returned. With most of Europe held hostage, they're unnecessary. Puppet regimes don't even necessarily have to know they're puppets and their nations are hostages, as long as they cooperate just enough.
Having achieved security beyond his greatest dreams, Stalin manages to settle down and sets to the work of achieving communist utopia, knowing that this will secure his conquests far better than any number of garrisons. He dies in 1970, beloved by millions across the world, his face on countless t-shirts. Of course people will argue bitterly that he was just as bad as Hitler.
Each side spies on the other at every chance, inserting agents and supporting dissidents at every opportunity, but breaking the status quo is too expensive for either side. After Europe has burned so hot for so many years, things settle down. The war becomes a cold one, if you will.