dstuva and mwestwood make very important points about the presence of very dangerous flaws in Brutus, Antony, and Cassius, and I do agree that Brutus is the most noble of the three. If given a choice, I might choose Antony as a leader; I think he has the personality best suited to rule in Rome during that time period. Though I'd like to think that Brutus would be the best leader, I think that his failure to make appropriate decisions would be his downfall. For, as dstuva notes, he makes numerous fatal mistakes. (The last occurs in Act 5 when he and Cassius disagree about battle strategy. As is the patter, Cassius is right, but he caves to Brutus's point of view.)
One of the great ironies of Caesar's having been assassinated is that while he desired power, he did not possess the dangerous flaws of Brutus, Antony, and Cassius that Post #2 so cogently points out.
Perhaps, Brutus, "the noblest Roman of all," would have made a better ruler than the others if he had been placed in this position under different circumstances. For, it seems that Brutus does love Rome so much that he is greatly disturbed by the devastating consequences of his action of slaying Caesar--the terrible civil war. For instance, it is clearly evident that he suffers from his guilt since Caesar's ghost is in his dreams. Thus overshadowed by his debilitating guilt about Caesar and the death of his wife, Brutus loses faith in himself and in his friend Cassius, who has persuaded him into this fateful matter. He, then, makes tragic mistakes in judgment, mistakes that may have been avoided if he were working with a clear conscience.
The answer to your question concerning Shakespeare's Julius Caesar is very much a matter of opinion. Each of the three characters you ask about has qualities that could make him a capable ruler.
Antony is intelligent, an excellent speaker, brave, ruthless when he has to be (could be a positive or a negative), and understands politics. He manipulates the crowd into doing his bidding. He could certainly rule.
Cassius understands politics, too, and is also intelligent. Had the conspirators followed his decision-making instead of Brutus's, they might have stayed in charge of Rome. But he is motivated by jealousy and personal gain. He takes bribe's in return for favors.
Brutus is the most noble of the three, but he is naive and a bad judge of character. He also makes poor decisions. He is intelligent, also, but doesn't understand human nature. He talks the conspirators out of killing Antony along with Caesar, and he talks the conspirators into allowing Antony to speak at Caesar's funeral: both fatal mistakes.