So you were the imprecise one =] gotcha.
The game opens with you seeing the battle of the Titan against the boss so even though there is a tutorial before you face the boss yourself, it is part of the opening sequence.
Opening usually refers to the first level of the game, not the first minute. If that were the case the choices here would be far more limited and many of the bosses in the poll wouldn't even be there.
I don't think this is even up for debate. It's Metal Gear Ray from MGR: Revengeance, and it's not particularly close.
Looking at the comments here, it looks like this is a VERY debatable topic and you're losing this debate as you're the only one who mentions Metal Gear Ray :)