First of all, Julius Caesar is seen as perhaps the weakest of these four characters. He suffers seizures; in the swimming match with Cassius, he cries,"Help me, Cassius or I sink" (Act I, Scene 2), and he wavers between superstition and ego when his wife tries to persuade him not to go to the Senate. However, he is a cunning man, much like Antony and Cassius. He refuses the crown to appear unambitious. Cassius is extremely ambitious and is a main instigator in the Conspiracy. We also find out he is somewhat greedy. Antony is clever and wants power, yet he also wants the good of Rome. He is a brilliant orator and is liked by the people.
Of all the characters, Brutus is motivated by good. He does not act out of ambition or greed, but for what he believes is truly the good of Rome! Brutus is also liked by the Romans though Antony changes that. Cassius and Brutus both commit suicide; Caesar is assassinated; Antony lives to join the Second Triumvirate! (A quick answer to a very big question as you could come up with lots more. Look further in the enotes links!)
I was skimming google and found this. I know its old but could not help but bring this up.
Brutus was not motivated by the good of Rome, but was concerned with being the perfect Roman and living a perfectlife;not so that he could be a good man, but so that he could be admired. Brutus craved loyalty and love, yet would only deny himself love. He shows his true colors in his eulogy when he never even mentions the well-being of Rome.