It's a subject that is far too complex for a poll or even a simple answer. However, if backed up against a wall and forced to choose I would put up two candidates - Germany and Russia. The case against Germany is well known, so I'll indict Russia...
1) Russia encouraged Serbian nationalism in a deliberate attempt to extend its influence in the Balkans at the expense of Austro-Hungary. These nationalist feelings spawned organisations such as the Black Hand and led ultimately to Sarajevo.
2) It is often accepted that during the crisis Russia 'had' to support its client, Serbia, against Austro-Hungarian threats. This is far too simplistic a view. If Serbia had been crushed then yes, there would have been a huge loss of Russian prestige in the Balkans, but Russia's vital interests were not at stake. This contrasts with Austria, whose very existence was threatened by the the assassination.
Even if Serbia had been emasculated, Russia could possibly have reverted to supporting Bulgarian ambitions in the region, which it had done until the Second Balkan War. In addition, Russia's Great Military Programme of reform and expansion would have tilted the balance of power in its favour by around 1917. It had no 'need' to act precipitately.
3) The step which made European war (as opposed to a local conflict) inevitable was Russian mobilisation. The process began on 26th July when Tsar Nicholas II ordered the 'Period Preparatory to War'. As official Russian documents admit, these preparations were to be carried out in secret - masked by diplomatic obfuscations - in order to lull the enemy into a false sense of optimism that war could be avoided. During this period Russian ministers and diplomats lied brazenly about the military measures which were taking place. Additionally, when Russia published its official documents shortly after war started, the date of mobilisation was changed to come after the Austro-Hungarian mobilisation rather than before - making Russia look innocent.
On the 29th, Nicholas ordered general mobilisation but rescinded this shortly afterwards following confusion over a newly-arrived telegram from the Kaiser appealing for peace to be maintained. When ordering the stand-down, Nicholas stated that 'I will not become responsible for a monstrous slaughter', thus demonstrating that he knew what Russian mobilisation would lead to. Under pressure from his ministers and generals. the Tsar ordered general mobilisation once again on the 30th. Faced with this, Germany had to respond and on the 31st an ultimatum was sent to St Petersburg; when Russia declined to back down Germany declared war on the 1st August.