Hitler made his two biggest mistakes in 1941 IMHO.
1) Decision to invade the USSR, a country too big to swallow especially when you consider that the Germans were fighting on other fronts.
2) Decision to declare war on the US just days after Pearl Harbor in December 1941. What exactly was Hitler thinking taking on a vast industrial power such as the US protected by two huge oceans? Did Hitler think his U-Boats were going to penetrate the Great Lakes? Did he think the Luftwaffe was going to strike American cities? Did he think his panzer divisions were going to roll over the fruited plain? What EXACTLY was Hitler thinking went he entered the Reichstag just days following Pearl Harbor to declare war on the US? At a time when thousands and thousands of his troops were literally freezing to death deep in the USSR.
Hitler visited Napoleon’s tomb before invading the USSR... he was arrogant.
Hitler was stupid NOT to provide the German armies with the winter uniforms, etc.
Hitler foolishly thought that the Japanese would reciprocate by declaring war on Russia and opening a Siberian front — foolish considering the spanking that Zhukov had given the Japanese during their ground war against Russia years earlier.
Hitler was fooled by Canaris, who sabotaged negotiations with Spain, which could have led to the Germans overrunning Gibraltar.
If Hitler was a “genius”, his megalomania, paranoia, and overconfidence effectively negated any genius.
I read a book that I thought was interesting (My apologies; I cannot remember the author, I believe the title was “How Hitler Could Have Won WWII). The premise of that book was that Hitler should have avoided war with both the Soviets and the US, and that he should have heavily fortified Afrika Korps to the point where it would have soundly defeated the British forces and conquered Egypt. From there, German forces could well have invaded British holdings in the Middle East and threatened India. Coordination with the Japanese forces in Indochina would have been possible here. Britain could not have easily defended India from a two-sided attack, and Britain would have almost been forced to capitulate to protect its colonial empire. The Soviets could have done nothing since they would have been faced with a direct invasion of the Caucus oil fields, which were a primary target of the German army in the actual invasion. Besides, it was the direct invasion by Germany that brought Stalin into the war in the first place. Absent this direct invasion, it’s unlikely that the Soviets would have done much.
Like any “what if” scenario, we’ll never know if it’s true, but it’s certainly an interesting idea.