Who is the best orator in Julius Caesar?

On the whole, one would have to say it’s Mark Antony. Brutus is a pretty impressive orator in his own right, and manages to get the restless mob on his side, but not for long. Once Mark Antony takes to the speaker’s rostrum he completely changes the common people’s mood, subtly turning the plebs against the conspirators who murdered their beloved Caesar.
Brutus has been a little too complacent in allowing Mark Antony to speak to the people. In his naivety, he seems to think that Antony will play ball and acknowledge the new order of things. After all, he hasn’t shown any defiance towards the conspirators since Caesar’s assassination.
But Mark Antony understands the power of oratory, and knows full well that he has that power. He doesn’t come right out and attack the conspirators; he’s careful to make it seem that he’s simply making a funeral oration, not a political speech:

Friends, Romans, countrymen, lend me your ears. I come to bury Caesar, not to praise him. (Act III Scene ii).

By praising Caesar so lavishly, Mark Antony isn’t departing from the traditional conventions of funeral orations; he’s simply doing what’s expected of him. But crucially, Mark Antony undermines the conspirators, not directly, but by the indirect method of reminding the plebs just how much Caesar did for them and how much they loved him. Antony’s speech is so effective, that by the time he’s finished, the common people have been roused to a fever pitch of righteous anger, determined to avenge the ruthless murder of their fallen leader.