The German Empire was the most blamed country of the war. They were the closest ally of Austria-Hungary. The Germans also supported Austria's invasion of Serbia for the assassination of Franz Ferdinand. The German Empire was the main opposition of the Allies since they were the ones that invaded France, Belgium, and Russia. The German Empire was also a pretty powerful aerial, naval, and land power, rivaling Britain in all of these theaters of war.
Austria-Hungary was a dying empire even before the war started. Many of their territories already gained their independence such as Romania. When their archduke was assassinated, they blamed Serbia for his murder. Austria-Hungary was the main opponent of the Balkans and Italy. Austria's chief of the military was not the smartest though. When Russia got ready for war faster than Germany thought they would, they asked Austria to deal with it, which work great if great means losing 1/4 of your territory.
The Ottoman Empire was divided on whether to join the war or not, then some of the politicians that wanted war went to Russia with some battleships and fired shells at them, so they joined the Central Powers. The Ottoman Empire posed a serious problem to the British because they could invade the middle east and take all of the British oil. However, the Ottomans didn't pose to much of a threat because they failed to actually invade both the Suez Cannel and the Coclasses mountains.