Historians will differ vigorously over this question for more than one reason.
First, there is the issue of whether individual human beings really have much of an impact on history at all. Some historians argue that it is really larger forces like economics and geopolitics that cause things to happen, not individuals. Similarly, some scholars of international relations argue that nations act in their own interests, regardless of who their leader is. In this view, WWII was not caused by Hitler but rather by Germany's desire to gain power and the Allies' desire to prevent Germany from doing so.
Second, even if you allow for the idea that individuals can impact history, you could still argue that Hitler wasn't the main cause of the war. You could argue that the real cause of the war was the Treaty of Versailles. In this view, the war was caused not by Hitler's desires but by the fact that the Treaty of Versailles was so harsh to Germany that it made Germans much more likely to go to war again to escape from the treaty's provisions.
My own opinion is that the war would likely have happened even without Hitler. Germany was likely to want to push the French and British very hard over the Treaty and those two countries were likely to fear Germany no matter who its leader was. It could be, however, that Hitler made the war come faster than it might have because he was so aggressive in his attempts to get Germany back to what he saw as its rightful place among nations.