Germany

Empires were dying doesn't means their armies were dying too, Ottoman Empire kicked British Imperial and Commonwealth ass at Galipoli and Kut brutally, Assaults on Jerusalem were slaughters for Egyptian Expeditionary force till 1918 when finally Arab Revolt cut their communications and supply lines.
Austro Hungarian army was still in better position than Italian and Russian. Austrian army was equally dangerous as Germans when under German or competent Austrian command.
French and British tactics on Western Front were just of pounding trenches for days mostly wasting large amounts of shells and alerting the Germans of the position where they were attacking, allowing them to bring reserves and then launching literal human waves against heavily fortified machine guns posts, ( British and French did not thought machine guns were of any use as late as 1916).
Germany has better industrial capability than British and large part of French industries was under German occupation and there was no lend-lease.
Germany proper has more manpower than any other nation in Europe except Russia which again do not have industrial capability to utilise them and nor it had capability to sustain them in army and it couldn't get any substantial help till Baltic was blocked by German Navy and Black Sea by Ottoman Navy.
Austrian Navy was anything but incompetent.
Central position of Central powers allowed them to have higher degree of cooperation.
German army was large and highly trained and equipped even during peacetime compared to largely raw conscripts and "volunteers from colonies" of British Empire. Though French have large army and it was "trained" and well equipped, it has outdated training completely ignoring existence of modern weapons like Machine guns and Heavy Artillery.
Bulgarian army was unusually large for a small nation, having largest army in entire Balkans.