Hitlers biggest mistake was trying to take over too much too fast. He should have done it the way russia did. slow and steady, giving him time to build the equipment he waould need for the next leg. But no thankfully he was stupid and wanted to attack every one and thin out his resourses to a point that he could not manufacture it fast enough to keep up.
Actually he did invest in the development of hvy 4 engine A/C. It was a mistake, they would never gear for mass production. That also would be a mistake, Strat bombing was arguably a waste of time for the allies(the way they did it most of the time anyway), it certainly would have been from germany.
As soon as Hitler had nearly won a battle, he lost interest in it and went off on another tangent. This allowed his enemies to recover, and eventually defeat him. There must be a word for this.
His big mistake was not listening to general Rommel (probly spelled that wrong) especially with the atlantic wall
That may have also helped Hitler. If he had been more casual and slow, that would have given the allies more time to build up there defenses. By quickly building up a powerful military and using it fast, he didn't give nearby countries much time to prepare. The U.S.'s extra help is what saved the allies asses.
The bottom line is Hitler made many stupid mistakes...including the tyranny that ravaged the land and the persecution of Jews, homosexuals and dissedents. His military blunders are legion...it is not just one thing. Any one thing or mistake would have been survivable. But together, they were just too much and not sustainable. So the biggest mistake hitler made was to be hitler...act like hitler and thinking like hitler.