The answer that is most likely to be the right one is: "Brutus says he was ambitious, / And sure he is an honourable man."
What Antony means with those lines is that, even though Brutus is an honourable man, a man with strong morals, he has misjudged Caesar's character. Brutus accuses Caesar of having been ambitious, but that is clearly not Antony's opinion. Caesar brought captives home, cried with the poor, and refused a crown three times. As Antony says, "Ambition should be made of sterner stuff."