Well, first of all, I don't think you have to worry about connoting praise by saying he was important. Important is neutral, indicating that he had an impact and cannot be ignored, and doesn't indicate whether his impact was positive or negative.
Without Hitler, there definitely is no European war in 1939. His stated goal was to launch a war of conquest against the Slavic East by that time, and if he himself did not exist, some of the other trends in Germany may have carried forward uninterrupted. I think Germany may not have avoided an authoritarian government of some kind, based on the weaknesses inherent in the Weimar constitution, and the power of the conservative/monarchist elements. On the other hand, the Social Democrats and Center formed a counterweight to those groups. As long as political discourse remained polite and peaceful, I don't think Germany lapses into a totalitarian police state on the order of the Third Reich. And without Germany's war of aggression against her neighbors, the conflict between the Soviet Union and the Western European states becomes the main tension, with something like the Cold War settling in.
On the other side of the globe, however, whether Hitler lived or not has little influence on whether Japan and the US go to war. Japan's leaders had designs on the Pacific that conflicted directly with American interests, and the likelihood of open conflict was high. That was expected by some experts ever since the Russo-Japanese War in 1905, and certainly by many after the First World War, when Japan's position was improved by taking over many former German possessions.
