In the end, Hitler was Germany's worst enemy. In the hands of a good military leader, Germany might very well have won the war. They came very close anyway. If he had waited a year or 2 to build up his submarine fleet before starting the war he could have isolated England completely. If he had continued his air attacks against the RAF for just a short time longer, they would have ceased to be a factor. Operation Barbarosa was another of his blunders. And so on, and so on.....
I guess we were lucky he was there......