Well, generally, many people think its the Germans (but they are confusing WW2 with WW1), when in reality, the Serbs were the ones that assassinated the Austrian Archduke Franz Ferdinand in which case, Austria-Hungary retaliated in which case, Germany joined in with Austria. But since there was a web of alliances, Russia came to the aid of Serbia but France was allied with Russia so it joined with Russia. But to invade France, Germany crossed over through Belgium which the British had an agreement with them so that forced them to come to their aid in the fight against Germany.
Note: This is only a small list of the countries involved
So getting back to the question, the bottom line is, Austria-Hungary was the aggressor even though Germany often gets blamed
Edit: Austria-Hungary is seen starting the fuse but you asked who "Many see as the aggressor" so I guess that would be Germany since many blame Germany since it invaded Belgium