I thought just for fun I'd do a list of the political assassinations that had the biggest consequences for the world. So here we go:

Gaius Julius Caesar

Hard to go past this one for the top spot.

Caesar's death led to the official end of the Roman Republic and the rise of the first Roman Emperor, Octavian Augustus, who just happened to be Caesar's nephew and heir.

I think we can reasonably say the Roman Empire was kind of a big consequence.

The Archduke Franz Ferdinand

A man defined by his death.

The otherwise forgettable Archduke managed to get himself killed by Serbian anarchists.  Which he largely did by ignoring not only a lot of serious warnings, but also a previous attempt on his life on the very same day.

Unfortunately, since he was heir to the Austro-Hungarian throne, his demise kicked off a war which in turn started a domino effect of treaties that ended with World War One.

So that's about 38 million casualties right there that this assassination caused, plus the near destruction of Europe.

Philip II of Macedon

He was the father of Alexander the Great. 

Philip was assassinated when Alexander was only twenty years old.  Alexander spent the next thirteen years conquering the entire known world, and then himself died.

The world would be a very different place if Alexander had spent those thirteen years as his father's lieutenant.

You might argue that Alexander would have gone on to conquer the world after he inherited the kingdom anyway, but Philip was only 46 when he died. He might have lasted another twenty or thirty years. Which would have left Alexander inheriting at age 40 or 50. 

So Philip's death at just that moment changed the world a lot.

Ephialtes

You've probably never heard of him, unless you've read my first murder mystery, which is about the death of this fellow.  

Ephialtes created the first true democracy at ancient Athens, which in turn invented the whole idea of Western democracy. Not a small thing.  

Ephialtes was promptly assassinated for his troubles, and here comes the part that makes his killing so significant:

Ephialtes had a lieutenant, a rather likely lad by the name of Pericles.  

Pericles took the top job when his friend died, and that was the start of the peak of classical civilization that we call the Age of Pericles.  

Carloman

Charlemagne had a younger brother, which was very inconvenient because by the rules of inheritance at the time they were required to split their father's kingdom.  

Charlemagne was particularly put out. He had plans to conquer Europe &/etc, and an uncooperative little brother was going to be a drag.  

Then Carloman mysteriously died, still a young man, in circumstances that were never explained, and no cause of death was ever given.

It was very convenient for Charlemagne though. He promptly conquered Europe and founded the Holy Roman Empire.

Which probably would never have happened if Carloman had hung around. Charlemagne was never actually accused of arranging the assassination of his little brother, however this must be tempered by the observation firstly that Charlemagne was incredibly good at planning things, and secondly that only a crazy person would accuse the Holy Roman Emperor of murder.

Do feel free to add your favourite assassinations in comments.  Somehow I have a feeling people will have their own lists.