I don't think one country was 'guilty' of starting WWI, because guilt would imply that actions were taken, pre-meditated, to start a continent wide war, which they were not. Hell, even the idea of German 'war guilt' largely stems from Weimar propaganda surrounding article 231, an insignificant article of the treaty of Versailles, in which the allies never actually said Germany was 'guilty' at all!

I do, however, feel that 'responsibility' should be assigned; if you hit someone with your car and kill them, but were impaired via drugs, alcohol, etc, you would be held responsible, as you are accountable for the death, but did not INTEND to kill the person.

The Austrians intended to crush Serbia, based on flimsy evidence that the Serbian State was directly involved in the assassination; the Germans issued the Austrians a blank check in this regard, assuring their support against Russia. When the Austrians issued their ultimatum, the Russians advised the Serbs to accept whatever terms were given; the Serbs accepted, Kaiser Wilhelm commented that there was now no reason for war ... and on July 28th, Austrian artillery shelled Belgrade!

The Russians wanted to avoid conflict, their government dealing with strikes across the country barely c.8 years after the 1905 revolutions had been crushed, and Russia's 'Great Programme' was barely a year old. The Tsar grudgingly mobilized first partially then fully, as a political move to convince the Austrians to back down and accept arbitration. 'Nicky' rightly informed 'Willy' that mobilization in Russia's case did not mean war, and trusted in the Kaiser to exert pressure on Vienna to back down. Edward Grey had already suggested a Four Power Conference in Geneva, which Russian Foreign Minister Sergei Sazonov and French Foreign Minister Jules Cambon both thought was an acceptable solution.

Unfortunately, Austria had issued the Ultimatum to be unacceptable; they WERE going to war, and nothing would stop them. At the same time, Helmut von Moltke the Younger, chief of the Greater German General Staff, had already drawn up the ultimatum that would be issued to Belgium, informing them of Germany's intent to use their country as an avenue for the 'transit' of their armies. Russian mobilization was all the Germans, already in the early stages of their own mobilization, needed to justify their subsequent actions as a war of defence. Ultimatums, meant to be refused, were issued to France AND Russia, and on August 1st war began. The whole problem was that once GERMANY mobilized, their mobilization stages were so interconnected with national railways, that it was either GO TO WAR, or not. Going to war also meant invading Belgium, Luxembourg and France, REGARDLESS of what actions those states took.

Austria's refusal to accept arbitration ensured that conflict would break-out in eastern Europe; Germany's earlier decisions not to renew their plans for a single front war with Russia, ensured that the conflict would expand into western Europe; the expansion of the conflict into western Europe ensured that Britain would be involved, and Britain's involvement ensured that this would be a World War.

Further reading: Hew Strachan, "The First World War, Part One: To Arms!" Annika Mombauer, 'Helmuth von Moltke and the origins of the First World War"; "The Origins of the First World War: Consensus and Controversies & Diplomatic and Military Documents" Geoffrey Wawro, "A Mad Catastrophe" David Stevenson, "1914-1918" Margaret MacMillan, "The War That Ended Peace" Thomas G. Otte, "July Crisis" Fritz Fischer, "German War Aims in the First World War" Max Hastings, "Catastrophe" Luigi Albertini, "Origins of the War of 1914" Holger Herwig, "The First World War: Germany and Austria-Hungary, 1914-1918"

The real problem with WW1 is that there wasn't a single country that we can blame more for starting the war, since most of the countries that took part in it contributed to the start of the conflict.

Your belief isn't really a fringe one, since quite a lot of people see Russia's mobilization against Austria-Hungary as the spark that ignited the war(and as you said Russia's desire for exapansion). More to the point, some historians see Russia's defence of Serbia as a mere casus belli they needed to declare war on the central powers and gain territory.

On the other hand I personally believe that the most responsible country for the start of WW1 was Austria-Hungary, since its foreign ministry and its army showed Serbia and the rest of the world that they wanted to go to war no matter what demands the Serbians accepted after Franz-Ferdinand's murder(Fun fact: The Archduke was actually one of the only leading Austro-Hungarians who was against another balkan war), hence the ridiculous ultimatum they sent Serbia, which was obviously turned down.

Last but not least Austria-Hungary pressured Germany into the Blank Cheque at the start even though the Kaiser wasn't as pro-war as he was just before WW1 started, turning a balkan war into a european one.

So basically if I had to choose i'd say Austria-Hungary for obviously trying to pick a fight with Serbia and thereby igniting a World War.

Sources: The First World war, Hew Strachan; 1914-1918 David Stevenson; The Great War(a weekly documentary on youtube which is very interesting).

Edit 1: Grammar check

Edit 2: Sources

I'd agree that contextually Austria-Hungary may have lit the fire in a gas tank, but as stated it's hard to pin point one specific reason. Here are the four M-A-I-N reasons most historians can agree with:

Militarism: The arms race around the turn of the 20th century was a huge leap forward from the standardized line formation. Britain was in a full-on navy race with Germany for potential control over seas around Europe and the Atlantic. Machine guns were becoming a key factor in warfare. In simple terms, Entente and Alliance powers had new toys, and they just couldn't not wait to start using them to achieve political, military and geographical gains.

Alliance System: This is self evident, but then again the perfect pretext for a war-hungry Europe. It can be argued that the Alliance System was mainly a tactic so the superpowers would be dissuaded to enter into a direct war with one another, but the spark wasn't a declaration of war between the Entente and the Alliance, but between an Alliance faction declaring war against an ALLY of ONE of the members of the Entente.

Imperialism: A lot was changing and up for grabs in Europe in that time. The Balkan states were very new to the European theatre and fresh off a war with the Ottoman Empire, becoming easily bullied around by Austria-Hungary. As you mentioned, Russia was looking for access to the Mediterranean. France was dreaming to retake Alsace-Lorraine from the Germans. Germany was looking for more colonial influence in the world like its Entente counter-parts.

Nationalism: Mainly a movement which began in Europe around the end of 18th to the 19th century, it very much created an atmosphere of "Us vs Them". Old grudges between head of states now spilled over to the populace and became bitter rivalries. More importantly, nationalism in this sense gave an immense sense of pride to one's own culture and enormous amount of mistrust towards anything else.

The idea of Russia being the main cause does bring an interesting argument. I know for example that they did not respond to Germany's ultimatum to stop mobilizing troops or else it would be forced to declare war against Russia (and effectively sparking the defense alliances between the Entente and Alliance).