The answer to this can vary according to a person's point of view. It is also, of course, different for WWII in Europe and WWII in Asia/the Pacific.
In Europe, the one person most responsible for the war was Adolf Hitler. You could argue that the Treaty of Versailles was responsible for the war, but ultimately, it was Hitler who decided to try to expand Germany as aggressively as he did. That German expansion led to the war.
In Asia, there is no one person associated with causing the war. The general cause was the Japanese desire for empire. You can argue that the war was caused by the unwillingness of the US and European imperial powers to allow Japan to have its own empire. However, the more common argument is that Japan caused the war by pushing to gain an empire first in China and then elsewhere.