I don’t really see how Germany could have ‘won’ in Europe in the sense that they could have defeated the Soviets and successfully invaded Britain. The problem for Britain and the US (in the war or out) was that mainland western Europe would have been either Fascist or Communist. Remember, the reason why Europe was the priority over the Pacific (once the US was declared war upon) was the threat of Communist Russia usurping Fascist Germany. (i.e. the second front, when it came, was a land grab).
Many posters have spoken about the technological edge Germany may have developed but it is important to remember that in almost every case Britain was on the same track. Also, the Battle of the Atlantic had been won with the aid of new technologies like sonar, radar and better Naval, Air Force and convoy tactics. Enigma was still providing a distinct advantage and the Royal Navy guarded the Channel with vastly superior resources.
In addition, North Africa (Brits, Australians, New Zealanders, South Africans, etc) was increasingly secure as was the mid-East, Russia would continue to absorb Germany’s resources and - crucially - have prevented access to the absolutely essential southern oil fields. In many senses, Germany was surrounded and the debilitating effect of limited raw materials would have kicked in at some point. By contrast, both Russia and Britain had almost limitless access to supplies (Russia though its own resources and Britain via the Empire and ‘friendly neutrals’ - the latter, particularly in North and South America).
Certainly Britain and its Allies could not have contemplated a second front without very heavy assistance because it was a stretch to contain Germany from the eastern Med right around to Iceland while at the same time keeping the shipping lanes open and also guarding against invasion (with the Canadians). Not to mention (eventually) fighting the Japanese to a standstill in Burma (British and Indian troops with Chinese and Australian assistance).
Frankly, it was all a bit of an effort to keep so many plates spinning as it was.
The only thing I can say with a fair degree of certainty is that by declaring war on the US after Pearl Harbor, Hitler ensured that mainland western Europe would not fall under Soviet rule (something, I believe, he would have considered to be the ultimate mistake) - whether Hitler had the forethought to know that is an interesting debate in itself.
And if you really thing the Axis could have come up with the Atom bomb, I’d suggest looking at the size and resources committed to the Manhatten Project.