A German nuclear weapon during WWII is not ASB. It wouldn't be decisive, either, as it's been pointed out, because the amount of bombs that could be produced was very limited.
The biggest mistake of the nuclear project was the decision of using heavy water as moderator. Heavy water was very expensive and of slow production, even with the control Norway facilities. Even without the sabotage of said facilities, German scientists didn't believe they could have ready a nuclear battery in time for the end of the war, which made the project fall out of priority.
The focal point is why Heisenberg decided to use heavy water as moderator instead of graphite (as he had suggested in December, 1939). This decision was based on a research paper by Bothe on the efficiency as moderators of water, heavy water and graphite. Heisenberg noted that the paper was using a "slightly impure" graphite for the measurements, but he still decided that it wouldn't do it, despite being much more abundant and easy to get a hold of.
This decision has been enough to feed a conspiracy theory where Heisenberg took part in the project under threats and he attempted to sabotage it from within. Others think that it was just Heisenberg not being able to own up to his own mistakes.
In any case, if they had decided to use graphite to build a nuclear reactor, they would have had a significant advantage over the Manhattan Project. Not only the Manhattan Project started much later (although the preliminar research began in 1940), the nazis also counted with the people who had done the original research on nuclear fission.
It's often pointed out that the Germans had many errors in the conception of the differences between bomb and reactor and on how to detonate the bomb. But the same lack of adequate ideas was present in the Manhattan Project, and it was the development of the nuclear reactor what helped understand how to do it properly.
Now, being imaginative and using a bit of handwavium to obtain an absolute best case scenario, it could give the Nazi Germany an advantage. Only if the research, the development of the nuclear reactor, and the isotope separation went absolutely flawless, and they could obtain a bomb in slightly less time than it took the Americans. And only if used tactically.
The handwavium timeline would be:the Nazis develop a working atomic battery in late '40 early '41 and develop a prototype bomb by mid '43, they forgo a trinity-like testing in favor of actually using it in battle. By '44, they have other 2 bombs they can use in the Italian front and in the Normandy invasion. At this point the allies might decide to go use their own chemical weapons, and the Americans probably can see that Trinity is a waste of time and test their first nuke on the battlefield against Germany.
I am thinking that seeing how an invasion from Normandy is a no-go now, they can try create their new front by invading Spain, and probably Spain and France would become the battlefields of a limited nuclear exchange and not-so-limited chemical warfare. And the allies would end up giving a green light to the Operation Vegetarian.