Brutus is the most noble. He never acted out of self-interest. He always thought about the good of Rome. He didn't kill Caesar out of hate, but out of love for his country. He also wanted to limit the bloodshed as much as possible which is why he told the conspirators to spare Antony. Brutus isn't perfect, but morally he's quite sound.
Caesar is in the middle, because although he is ambitious, he also cares deeply for his friends and his country. He showed excess ambition when he killed off Pompey (before the play began). He also showed how badly he wanted that crown and how angry he was when the people cheered the fact he declined it. Still, people like Antony were loyal to him because they knew he was a great man. His will also showed his love for Rome and its people. When he realized Brutus was among his assassins, he was genuinely hurt because he took Brutus in as if he were a son (this too wasn't part of the play).
Cassius is the least noble. He operates purely on self-interest. He wants power and knows he'll never get it if Caesar is around because Caesar doesn't like him. He knows Brutus' weaknesses and exploits them to trick Brutus into thinking Caesar needs to die for the good of Rome. He proves to be a plunderer in Act IV and acted in the same corrupt ways he criticized Caesar for in Act I. Never once did Cassius do anything for anybody but himself.