Who do you think is the true hero (or protagonist) of Shakespeare's The Merchant of Venice, Portia or Antonio?

Your question is actually more complicated than it seems since it demands two interpretations of the term, 'hero.' In literature, the word normally refers to the lead character. Because the play is titled, 'The Merchant of Venice,' Antonio becomes the most obvious reference since he is the only merchant we read about from beginning to end. In this sense, he is, therefore, the protagonist.

Further support for him being deemed the true protagonist lies in the fact that all the major events revolve around his actions. The main theme of the play explores the antagonism between him and his opposition, Shylock, and delves into the origin and nature of their dispute, its development, and final resolution. The actions of most of the other characters are determined by events which he influences and by situations which have an impact on him. In essence, he is the core around whom almost everything revolves.

It is Antonio's decision to assist Bassanio by agreeing to act as surety that affects the major events in the play: Shylock's demand for restitution in the form of a pound of his flesh; Portia's decision to disguise herself as a lawyer; the trial scene. The major themes all have their origins in these particular situations. Although Portia's role is important, her actions later in the play are all affected by Antonio's dilemma.

In addition, your reference to 'true' suggests 'real' in the sense of, 'Who is the real hero in the play?' I suggest that this requires an alternative interpretation of the word. In this definition, a hero is someone who undertakes an arduous task and is admired for acting bravely in the face of adversity. A more common interpretation is a reference to someone who saves another from jeopardy. In this interpretation, Portia is a hero, for she saves Antonio from Shylock's vengeance. She becomes his savior and her deed is much admired and appreciated by her contemporaries.

Antonio, for all his magnanimity, does not accurately fit this definition. The fact that he extended assistance to Bassanio, against his advice, does not entirely qualify him. His attitude in accepting the harsh and punitive conditions of Shylock's bond was born from his naive overconfidence. He had no doubt that his merchant ships would dock safely and not come to harm. 

On the whole, you'll to weigh up the two definitions and determine which best suits the purpose of your task.