Personally, I think I would have to go for Cassius as being the most Machiavellian of the characters in this play of intrigue and plots. He is presented, initially at least, as being the most manipulative and persuasive of characters. Note how the famous "seduction scene" of Act I scene 2 demonstrates these qualities. In his speech, Cassius shows an incredibly shrewd understanding of Brutus and the best way in which to turn him against Caesar and to encourage him to join the conspirators. His soliloquy at the end of Act I scene 2 seems to demonstrate his ability to "read" characters incredibly accurately:
Well, Brutus, thou art noble; yet I see
Thy honourable mettle may be wrought
From that it is disposed; therefore it is meet
That noble minds keep ever with their likes;
For who so firm that cannot be seduced?
Thus when we think of Machiavellian characters, Cassius appears to demonstrate the most Machiavellian qualities. In a sense, Brutus is too noble and well-meaning. He appears as a character who is only manipulated by others, but Cassius appears to be the real "brains" behind the operation.