Since there is already a lot of threads about the most responsible country for WWI, and AFAIK we don't have any thread with this premise yet, so...
Germany, Austria-Hungary, Russia, and Serbia are certainly out of question...while countries like Japan, Italy, Bulgaria, United States, Romania, Belgium, Montenegro, etc, were either minor belligerences or entered the War lately (therefore they will almost certainly win this poll).
That leaves us with three major players: Great Britain, France, and Ottoman Empire.
Well, which one, then?