Antony is portrayed as the hero of the play, as opposed to Cassius and Brutus. Antony is incredibly loyal to Caesar, even before Caeser's death. In Act I, scene ii, Caesar asks Antony is touch Calpurnia during his foot race to cure Calpurnia of her infertility. Antony replies with, "When Caesar says, 'do this', it is performed."
Antony remains loyal to Caesar after Caesar's death. Combining his loyal with his ability to persuade the common people, Antony takes revenge on Cassius and Brutus. He is able to use his rhetoric (ability to speak and persuade) to incite emotion and action in the plebians. His words and speeches are able to drive Cassius and Brutus out of Rome and start a civil war.
Antony is also a shrewd warrior, which can be seen as a positive trait. He is using the civil war to dispose of enemies and gain power for himself. Historically, this became the source of Antony's downfall. The majority of the end of the play (acts four and five) focus on Brutus and Cassius, resigning Antony to a role as a follower of Octavian. However, at the beginning of Act Five, Octavian and Antony seem to have a similar power struggle that Brutus and Cassius had in the previous act.
Antony is a hugely ambiguous character. He hardly appears in the first half of the play at all, and only really comes into focus once Caesar has been murdered. All we really find out about him in the early part of the play is that he's something of a party-animal, and that he enjoys plays. And Cassius, who's always right in this play, fears him as a potential threat.
His good points only come into focus after the murder. First thing to say is that he's an extremely skilful politician. He knows to take the conspirators' hands, and he makes sure that he makes it seem completely uncalculated:
Gentlemen all,—alas, what shall I say?
My credit now stands on such slippery ground,
That one of two bad ways you must conceit me,
Either a coward or a flatterer.
That I did love thee, Caesar, O, 'tis true!
He is, of course, a flatterer. A complete flatterer. So much so that the audience actually believe that he's on the side of the conspirators. It's only when he makes his speech that you think he might be genuine:
O, pardon me, thou bleeding piece of earth,
That I am meek and gentle with these butchers!
And then, of course, his famous speech at the funeral turns the tide against the conspirators, and makes the act of doing so seem uncalculated. He's an amazing speaker. In fact, he's such a good speaker, that you never know whether to believe him. Is he a good friend - is he loyal to Caesar - is he self-serving? Depends which of his lines you buy and which you don't.