Okay, so I'm prefacing this with, the Nazis lose the war anyway, we all know that, this is just more of a "I wonder if one small change could have made major changes to the war?" question. And I came to one name.
Juan Pujol García.
If you aren't familiar with him, he's a Spanish man who decided in World War 2, he wanted to help the Allied cause. So he went to the British embassy and asked if he could help out. They said no.
So he said, no problem, I'll do it myself. (But probably in Spanish.) Man creates a fake identity as a fanatical fascist. Goes to the Nazis. Says he wants to help them out. They say, absolutely, go to Britain, start spying for us.
He moves to Lisbon. Collects travel guides and train schedules and newsreels and magazine ads about Britain. Starts spinning fantastic bullshit to feed the Nazis. Anyone who knows Britain would recognize he's lying. The Nazis think he's great. Tell him to start building a spy network. So he starts making up names to create some imaginary spies. They've got backstories. At one point he says one of them died to excuse the delay on some news.
British intelligence, meanwhile, is hearing about the Nazis getting the reports from some network of spies hidden in Britain. Start searching furiously. Finally get their hands on the reports the Nazis are reading and... Realize this is all fake.
Short version is, the British find Garcia, bring him over to Britain, and he feeds the Nazis fake intelligence for the rest of the war- well, some of it's fake, some of it is true that is just delivered a little too late. It all leads to one conclusion: Germany loved him. Awarded him the Iron Cross for his service. Notably, to award the Iron Cross to anyone who didn't fight on the front lines required Hitler's personal approval. The service was, of course, over the radio. Meanwhile Britain loved him even more, for obvious reasons- four months after he got the Iron Cross, Britain named him a Member of the Order of the British Empire, one of the few people in the war to receive high honors from both sides.
Now, what I've rambled to here is: I've read speculation that the constant reports from Garcia helped the Nazis feel like they didn't need to invest more on infiltrating the British, so his counter-intelligence operation was incredibly effective, especially for something that started with a guy doing it all on his own initiative out of an apartment in Portugal.
So, it's obviously kind of an impossible question to really get to details on, but it's something to wonder about: What if he never started doing this? What if the Nazis didn't have this source for what they thought was reliable information in the UK, and made more efforts to infiltrate and spy on the British war effort?