The most plausible scenario I can come up with is this: Germany wins the Battle of Britain, and prepares to launch an invasion, fortunatley the navy led by the good ol' Hood crushes the German invasion force, so Germany must now turn to cutting Britain's supply lines with whatever they have left, likely, the Lufwaffe keeps bombing British industrial targets to prevent the rebuilding of the RAF, and Germany's commerce raiders would continue attacking the British convoys, while the Lufwaffe would likely turn to the British navy, they would do a Pearl-Harbor style attack on the British naval bases and then, with much of the navy in tatters, the Germans could send another fleet led by Bismarck and Tirpitz to land.
They would likely bombard a british port until it surrendered, and then they would land troops in it. Afterwards, they could continuously ferry troops across while the Lufwaffe does its job. Britain was a sea power, not a land power, so it would have likely been defeated by mid 1941 in this strategy.
Afterwards, the Germans could turn to the eastern front, and send most of their available army to attack the Russians. With more troops available, then perhaps Barbarossa could be successful, though it was really Hitler's decision that Guderian's panzers assist the army in taking Kiev that saved Moscow, if that didn't happen, then Moscow would have fallen and perhaps Stalin, like Hitler, would commit suicide in his bunker.
With Russia neutralized, then Germany would indeed be victorious, and they would likely assist Japan in its war against China. But still, there would be one thorn in the side of the Axis.
Hitler felt that America was the most dangerous of the three possible enemies he could face, coupled with its vast manpower reserves, strong navy, and industry, then a fight between the two would have been disastrous.
Even if America had a small army, they always have raised enormous armies when needed, in the civil war their small army went to 2 million strong in the entire war, with 500,000 at the beginning, at the same time, in World War I, they went from 28,000 to 3 million.
So if America faced Germany, then it would have been a fight that was anyones game, plus, Canada and many other nations would rally behind the US to defend themselves, and the combined might of that might be enough to defeat even the Nazis.