The protagonist in The Merchant of Venice by William Shakespeare is often considered to be Antonio, as he is the character through whom all the other characters in the play are connected.
Antonio is a merchant in Venice (note the title of the play). He is rich and makes grand investments buying and selling goods from all over the world; he also loans money at no interest, which certainly does not endear him to the Jewish moneylender, Shylock. Bassanio is Antonio’s friend, and through Bassanio we meet Portia and all the people connected to her plot line.
It is true, however, that Shylock has many of the most-quoted and most-remembered lines in the play and a case might be made that he is the protagonist; he is the one who takes action while Antonio takes a rather passive role. Despite that, he is not involved in the resolution of the play, so perhaps he is not the best choice.
Portia also drives much of the plot in this story, and she is there from beginning to end. While she claims she is trapped and cannot make her own choices, she certainly manages to get a lot accomplished either directly on her behalf.
While Antonio is certainly antagonistic toward Shylock, Shylock is generally considered to be the antagonist of this story. When Antonio cannot pay his debt to Shylock, Shylock takes him to court purely for spite and revenge. When asked by the court why he is doing so, he answers:
So can I give no reason, nor I will not,
More than a lodged hate and a certain loathing
I bear Antonio, that I follow thus
A losing suit against him.
If Shylock is the protagonist, Antonio has to be the antagonist. While his behaviors are certainly antagonizing to Shylock, they probably do not rise to the same level of hatred as Shylock’s actions.
Outside of his cruel, anti-Semitic behavior toward Shylock, Antonio does display the admirable Christian qualities of generosity, kindness, and friendship. He is a rather complicated character with at least one egregious character flaw, but he could be the protagonist in this play.