In Shakespeare's Julius Caesar, who led the assassination of Julius Caesar?

Cassius was a noble Roman who initiated the conspiracy who plotted the murder of Julius Caesar. Cassius influenced many other noble Roman leaders to join the conspiracy. Cassius influenced Brutus who was considered the most honorable:

Brutus, I’ve been watching you lately.
I haven’t seen that gentleness in your eyes
And friendship that I used to see.
You’re too stubborn and too strange
To your friend that loves you.

Cassius feels that Brutus is troubled about something. Brutus admits that he is "at war with himself." No doubt, Brutus has been troubled about Caesar showing signs of ambition. Brutus joins the conspiracy because he fears that Caesar is becoming too powerful. Brutus becomes a leader of the conspiracy.

Brutus loved Caesar, but he feared that Caesar was becoming too ambitious. Brutus feared that Caesar would crown himself king and make slaves out of the Roman people:

What does this shouting mean? I’m afraid the people
Are going to choose Caesar for their king.

Cassius questions that the honorable Brutus would not have Caesar become king:

Yes, you fear it?
Then I must think you wouldn’t want it.

Then, Cassius begins plotting ways to ensure that Brutus will indeed join his conspiracy to kill Caesar. Brutus is truly considering joining the conspiracy:

I’m not angry that you’re my friend;
As for what you’re persuading me to do, I have some plans.
I’ll tell you later what I’ve thought about this
And about these times,

When Brutus fully commits, Brutus begins to lead the conspirators in the murdering of Caesar. The conspirators had so much respect for Brutus. Even Caesar loves Brutus. When Brutus stabbed Caesar, Caesar's last words were:

Et tu, Brute? Than fall, Caesar!

This comment means, "You too, Brutus? Then I must fall. Caesar had that much faith in the honorable Brutus. If Brutus were stabbing him, then Caesar must deserve to die. Caesar loved Brutus. He felt that Brutus must know something about his character that even he himself did not realize. Perhaps, Caesar had become too ambitious. Perhaps, even Caesar did not recognize that he was changing and becoming too powerful for his own good. 

In Julius Caesar, who masterminded Caesar's assassination?

Cassius plays the major role in organizing the conspiracy that results in the assassination of Caesar. Cassius is clever in manipulating Brutus to join the conspiracy. He works on Brutus psychologically, appealing to his love for Rome, his sense of honor, and his ego, as well. Cassius emphasizes how great Caesar's power has become--a threat to Roman freedom--and he insists that Brutus is certainly as worthy as Caesar as a man and a leader. To further advance his purpose, Cassius plots with others already a part of the conspiracy to play upon Brutus' sense of responsibility to the people of Rome. He forges letters, supposedly to Brutus from Roman citizens, and arranges for them to be thrown into Brutus' window and displayed in the city. Cassius is sure of himself in his manipulation of Brutus. He tells his fellow conspirator Cinna the following:

Three parts of him

Is ours already, and the man entire

Upon the next encounter yields him ours.

Cassius' prediction is correct. Brutus does join the conspiracy, Caesar is assassinated, and Cassius' hidden agenda is achieved.