Is Antonio or Shylock the main character of "The Merchant of Venice"?
It's an odd thing, sometimes, that Shakespeare plays take on a baggage which is more or less separate from the text itself. Othello, in recent years, has become all about Iago: and then Michael Grandage at the Donmar Warehouse did a production with Ewan McGregor that made you realise the play was actually structured entirely around Othello (as the title suggests).
And I think the same thing genuinely is true of Antonio. The play opens by raising the question of his sadness, it follows the story of his investment, his oddly homosexual attachment to Bassanio (is this his sadness? I think so...), his trial, and then his supposed redemption in the last act. He's certainly the character who gets the most stage time and, I think, speaks the most lines.
Shylock, on the other hand, is only in five scenes. Post-Holocaust, his story has taken on a particular resonance: but the play drops him unceremoniously out at the end of the fourth act, and he doesn't really get the chance to garner any sympathy.
Can you make an argument that Shylock is a Merchant of Venice - though, ironically, not culturally 'of Venice' as such, but an alien in Venice, if you see what I mean? Well, perhaps. But I maintain the structure of the play puts more eggs in Antonio's basket: and a really innovative director would make it work like that onstage, too.