Why does Antony call Brutus "the noblest Roman of all"?
Antony realizes that Brutus, almost alone among the conspirators, and really, the audience might think, among all of the characters in the play, acted in the interest of Rome. All of the others acted out of jealousy of Caesar's popularity and power or out of personal ambition. Brutus killed Caesar because he believed him a threat to Rome itself:
All the conspirators, save only he,
Did that they did in envy of great Caesar;
He only, in a general honest thought
And common good to all, made one of them.
Brutus's suicide at the end is thus a fitting end to a man whose actions in the play are self-sacrificing in nature. Antony's comments upon finding Brutus's body form an interesting counterpoint to his speech over Caesar's body. Then, when he referred to Brutus as an "honourable man," he was doing so sarcastically. At the end of the play, he does so literally.