Interesting topic. I think talking just pure battlefield stuff the allies would have won. Europe isn't the only theatre to consider, the allies had a huge presence in the Pacific and could have created a 2 front war coming from the east (though a long trek to get to anything worthwhile) or up from the Middle East. Strategic bombers could have launched from East Africa and the Middle East as well. There's also the possibility of amphibious landings in the Baltics. By 1945 the allies were well versed in multi front wars and amphibious landings, and had access to approach Moscow from multiple directions. The Germans did not. It would have been a long, bloody, costly war but I think the allies would have won even without using nukes. The recovery would have taken decades.
Where this all falls apart is the home front. If the allies were the aggressor it would have been a VERY unpopular war. Just as the war is coming to a close and it looks like our soldiers will be coming home, our leaders decide to attack a country that has fought side by side with us (metaphorically) for years. No way the public would have supported it. morale would have plummeted. Probably riots, possibility of desertion. Guaranteed none of the politicians that were in favour of it would be around after the next election. Had the Soviets made the first strike that would be a whole different story. An ally stabbing the rest in the back would have produced a similar reaction as Pearl Harbour, and the home front would have rallied.