The Franz Ferdinand one is hard to quantify really bc you can reasonably assume within 5 years, something else would have set off a very similar chain of events, but at the same time Ferdinand's killing is what actually did it, and if even one power was swayed otherwise by some different event that started the conflict, the outcomes could have been so drastically different
The assassination was carried out by the Young Bosnia cell, which of course tied itself to the Black Hand. A Serbian Army organisation without legitimate grassroots behind it, the Black Hand (let's not forget its real name,
Unification or Death) had the purpose to further the goal of creating a Serb-dominated ethnostate out of the Austro-Hungarian-controlled Balkans. And if you know anything about Serb ethnostates, that absolutely means genociding the native peoples of that region.
The motive for the assassination was to prevent Ferdinand from eventually enacting his planned policies of granting autonomy to the various Balkan Slavic peoples inside the Empire, because this would threaten their idea of a Serb-controlled Balkans. Gavrilo Princip himself even admitted this. If Ferdinand lives to see power, you get a significantly defused "Balkan powder keg", which doesn't mean no war, but it possibly means the First World War as we know it, with this specific set of alliances, never happens.
(I've always been a believer that the UK/US/France group fought the wrong enemy in WW1, as the situation as it happened and chain of alliances that resulted tied us not only to Serbia, but to the Russian Empire, another pan-Slavist genocide state whose support of Serbia started the chain of wars and without whose strength and territorial extent the world would be a far better place)
