I myself being Serbian, get the "you started world war 1 you bastard!" remark thrown at me quite often. It doesn't bother me, it's merely a joke. I am curious to see what the majority of people believe.
Neither/Austria-Hungary.
There's a lot of redundancy between the two threads (as it's basically the same topic and I referenced the latter for the former) but it's two slightly difference focuses so I'll link both for a sure thing.
To summarize my thoughts:
Germany is most responsible as they were most directly responsible for creating the European power blocs (Russia/France/Britain & Austria-Hungary/Germany/Italy/Ottomans) and divisions which made war nearly inevitable. That is traditionally why they are given war guilt; it was their expansionism and policies which divided the world and made the proverbial 'powder keg' in the first place.
In a technical sense Austria-Hungary did declare war first but it would have only came if Germany gave them permission. Austria-Hungary delivered demands to Serbia that was tantamount to Serbia becoming a protectorate of Austria-Hungary and demanded every item on the list be accepted -- and they were all but one. She delivered that ultimatum knowing full well Serbia would deny it and it would mean war and Germany gave Austria-Hungary permission to do it knowing full well it meant war.
So blame is placed on Germany for creating the powder keg, filling it with powder, and handing Austria-Hungary the match. Austria-Hungary gets blame for actually dropping the match. Though this analogy breaks down when we also consider the fact that it was Germany who turned it from a local conflict (A-H vs Serbia & (maybe) Russia) into a world war by preemptively invading neutral Belgium (dragging Britain into the war) and then France bringing, well, everyone else into the war.
So either neither to your original question (because Germany) or both Germany & Austria-Hungary depending on your analysis. I'd hardly call Serbia responsible when they said no to a blatant expansionist move by Austria-Hungary.