Hitler made some really fucked up decisions during teh course of the Second World War, which I'm going to list below.

1. Before Germany declared war on Russia, it was imperative that Britain be defeated. If Hitler had not ordered the Luftwaffe to switch from attacking the RAF and instead bomb English cities(in retaliation for negligent British raids in Germany), the RAF would have been utterly destroyed within weeks and Britain helpless and unable to stop the Luftwaffe from doing what it wanted.
2. Not telling Mussolini to clean up his own fucking mess in the Balkans. Instead, German manpower was tied down for months, thus delaying the start of Barbarossa, which ensured the Germans would be caught in the worst winter Russia had seen for decades, without cold weather gear or winterized vehicles.
3.Declaring war on the United States. If Hitler had not of decided to go to war with the USA a week after Pearl Harbor, there was no way FDR would have convinced the majority of Americans to also go to war in Europe.
4. His insane refusal to allow German troops to withdraw/ This bullshit was responsible for the death and capture of millions of German soldiers. If he had of allowed strategic withdrawals when needed, the Russians would have never made it to Germany, and there would have probably been a negotiated peace on the Eastern Front, and his refusal to release needed reinforcements until well after D-Day insured the Western Allies would prevail on that front.
In many ways, Hitler was the best thing going for the allies.
The best thing that could have possibly happened to Germany in WWII was if Hitler had od died or been incapcitated early in the war.