First of all, the alliance system was important in starting the First WW.
Bismarck believed that it was the best way to isolate France. In the period 1882-1907, the Triple Alliance (Germany, Austria-Hungary, Italy) and the Triple Entente (Britain, France, Russia) were set up. The alliance system was important because it could turn a local war into a general war. If a member of the Triple Alliance was at war with a member of the Triple Entente, the other members had to help them. In this way, Germany who first started this bears the greatest responsibility for starting the First WW.
Second, Germany is extremely nationalistic. She always believes that the Germans are the most superior race and she has the historic mission to build a powerful and strong Pan-German state. Wishing to unite with the Germans in Austria-Hungary , she supported Austria-Hungary in the Bosnia Crisis and the two Balkan Wars. Without German support, Austria-Hungary would not have determined to declare war on Serbia and later Russia and others. In this way, Germany was greatly responsible for the First
WW.
Third, Germany competed with Britain, France and Russia in the Armament Race and was responsible for causing the First WW. She raced with Britain in building a large army, a large navy and in producing weapons of war. From 1910 to 1914, while France increased her defence expenditure by 10%, Britain by 13%, Russia by 39%, Germany was the most militaristic as she increased by 73%. Germany’s military power made her ready to fight the war.
To conclude, I think although other nations shared some blame, Germany was, to a large extent, responsible for the outbreak of the First WW by starting the alliance system, advocating Pan-Germanism and joining in the Armament Race.